Mission Status Reports - Ulysses
ULYSSES STATUS REPORTS
by R.G. Marsden,
Ulysses Project Scientist, ESA
The joint ESA-NASA Ulysses mission finally came to an end on 30th June 2009, a full year after the originally announced mission end date of 1st July 2008. On 30th June 2009, the final ground station pass for the mission took place over DSS-63 (DSN Madrid 70m ground station) from 15:35 to 20:20 UTC (08:35 to 13:20 PDT). An open-loop slew manoeuvre was time tagged for execution at 11:20 UTC on-board the spacecraft. This manoeuvre resulted in the spacecraft pointing directly at the Earth during the mid-point of this pass for maximum downlink margin. The final activities were as follows:
UTC Timestamp: 30-Jun-2009 18:23:
The last command was sent to the spacecraft. This switched the downlink into 64 bits per second engineering only.
UTC Timestamp: 30-Jun-2009 19:34:
The thruster catalyst bed heaters were switched off.
UTC Timestamp: 30-Jun-2009 19:47:
Switched from receiver one to receiver two.
UTC Timestamp: 30-Jun-2009 19:59:
The command counter confirmed reception of the final command.
UTC Timestamp: 30-Jun-2009 20:15:
The ground station was unable to find the carrier signal, confirming that the transmitter was switched off on the spacecraft. Goodbye Ulysses.
More than 1500 publications had been published by the end of 2009.
The spacecraft no longer has enough power to run all of its communications, heating and scientific equipment simultaneously. Predictions regarding the remaining operational lifetime of Ulysses made earlier in the year following the loss of the X-band transmitter in January 2008 have proven to be pessimistic: the science mission is still ongoing, despite the low on-board temperatures. This is largely due to the introduction of "fuel bleeding", whereby two oppositely-directed thrusters are fired simultaneously every two hours, causing hydrazine fuel to move through a short length of pipework where fuel is likely to freeze.
Spacecraft operations and science data acquisition will continue using this same strategy until no more fuel remains or freezing occurs. Because of the increasing distance between the spacecraft and Earth, the S-band downlink can be used for playback of recorded data for very limited periods only. In general, spacecraft data received in real-time during a tracking pass are all that can be acquired.
With the spacecraft still sending back scientific data, albeit at a slower rate, the mission is not over yet: October 2008 marked the 18th anniversary of Ulysses in orbit. On 31 December 2008 Ulysses was at 32o N solar latitude and 4.1 AU from the Sun. All data is publicly available through the ESA Ulysses archive at: http://www.cosmos.esa.int/web/ulysses.
Among the latest Ulysses science results is the finding that the solar wind is currently at its weakest since the start of the space age. The observed long-term trend of lower dynamic pressures implies that the extent of the heliopause - the Solar System's buffer against galactic cosmic rays - has been shrinking.
The 2nd Heliospheric Network Workshop was held on 5-9 May in Kefalonia, Greece.
More than 1450 publications have been published by the end of 2008.
Ulysses Communication Anomaly
17 Jan 2008 15:29
Early on 15 January, at the start of a routine test in preparation for the next phase of the Ulysses mission, communication with the spacecraft via the onboard X-band transmitter was lost. As a result, the Spacecraft Operations Team declared a Spacecraft Emergency in order to obtain additional ground station coverage from NASA's Deep Space Network (DSN).
The team was then able to send the commands needed to switch to the S-band transmitter, and establish stable communications, albeit at a low bit rate. Subsequently, the bit rate was increased to 1024 bps using a 70m DSN station and the data recorded onboard the spacecraft during the anomaly could be played back.
Since the spacecraft is in a very stable configuration and there are no power or thermal concerns, the Ulysses Project ended the Spacecraft Emergency status late on 16 January and Ulysses is now in "Safing Mode". Analysis of the problem with the X-band transmitter is on-going.
All instruments are currently switched on and functioning nominally. Nutation operations are proceeding as planned using closed-loop Conscan and SOLACE manoeuvres (open-loop slews together with Conscan) to maintain Earth pointing and damp nutation.
Perihelion passage occurred on 18 August, and the spacecraft is now in the northern helio-hemisphere. On 27 August, Ulysses was closer to the Earth (0.42 AU) than at any time since it headed towards Jupiter in 1990.
The spacecraft has completed its third South Polar Pass in April, having reached maximum southern latitude (-79.7º heliographic) on 7 February. The spacecraft subsystems and science instruments remain in good health, with no anomalies during the reporting period.
Operations and Archive
Nutation operations commenced on 14 February as planned, with the first signs of the disturbance (caused by asymmetric solar heating of the axial boom) appearing after an open-loop slew on 10 February. As in the past, closed-loop Conscan manoeuvres have been carried out as required to control the nutation, which is expected to be present until mid-February 2008. The ESTRACK ground stations at Kourou and New Norcia are being employed to fill gaps in coverage arising from incomplete spacecraft visibility from the DSN Canberra complex. Science operations have, in general, been conducted according to the agreed payload power-sharing plan. Some modifications were necessary in March and April owing to slightly lower-than-expected power margins. Ground segment performance for the period has been excellent. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses.
On 3 February 2007 Comet C/2006 P1 McNaught, the brightest comet observed from Earth in the last 40 years, was almost radially aligned with the Ulysses spacecraft, then at a heliocentric distance of ~2.4 AU and 79º south heliographic latitude. The comet was ~0.7 AU from the Sun. During a ~4.5-day interval (5-9 February), Ulysses encountered the tail region of this spectacular comet. The region of disturbance in the solar wind produced by the comet was nearly 10 Mkm wide at 2.4 AU. During the encounter, the speed of the solar wind dropped from ~750 km/s to a minimum of 360 km/s and the proton density dropped by more than 2 orders of magnitude. Simultaneously, very large fluxes of molecular and singly and doubly charged atomic ions of cometary origin were detected. The slowing, depletion and heating of the solar wind was a result of charge exchange with neutral atoms and molecules from the comet, and with the pickup up by the solar wind of the newly-born cometary ions. Although no shocks were observed during the encounter, the magnetic field strength was slightly enhanced in broad regions at the leading and trailing edges of the comet's tail. The field was generally weaker inside the tail than in the unobstructed solar wind and pointed nearly radially inward during much of the encounter, opposite to its normal outward direction in the southern polar hemisphere at this phase of the present solar cycle. There were, however, shorter periods when the field pointed nearly radially outward, indicating the filamentary structure of the comet tail. This is the third confirmed crossing of a comet tail during the mission (the others were comet Hyakutake in 1996 and comet McNaught-Hartley in 2000).
On 6 October, Ulysses will have completed its 16th successful year in orbit. The spacecraft continues its climb to high southern latitudes with all subsystems and science instruments in good health. By the middle of November, the spacecraft will have reached 70º south solar latitude, marking the start of the third South Polar Pass.
Operations and Archive
During the reporting period, science operations have been conducted according to the revised payload power-sharing plan. Largely as a result of the gradually improving thermal situation as Ulysses gets closer to the Sun, several instruments not in the core payload category have been able to acquire data for short periods of time (typically 1 month). These include the gamma-ray burst experiment and the solar wind electron sensor. Ground segment performance has been excellent, leading to an overall data return for the period of 98.6%. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses.
Rather than focusing on a new science result from the mission, this report highlights the many contributions to Ulysses science made by Johannes Geiss. As well as being one of the fathers of the Ulysses mission (and one of its longest-serving PIs), Geiss is a world leader and foremost expert on measurements and interpretation of composition of matter that reveals the history, present state, and future of astronomical objects. A symposium devoted to these topics was held in September to celebrate Geiss' 80th birthday. At that meeting, George Gloeckler, his Co-PI on the Ulysses Solar Wind Ion Composition instrument (SWICS), noted that Johannes Geiss was first to measure the composition of the noble gases in the solar wind when, in the late 1960s, he flew his brilliant foil experiments to collect solar wind ions on the five Apollo missions to the moon. In recent years, Geiss, together with his colleagues on the Ulysses/SWICS team, has determined the isotopic and elemental composition of the solar wind under all solar wind conditions and at all helio-latitudes.
Many of the fundamental contributions to our knowledge of the heliosphere to have been made using Ulysses/SWICS have come from the study of pickup ions. Johannes Geiss and colleagues discovered and characterised pickup ions of interstellar origin (in particular, heavy ions and 3He) from which the composition of the neutral gas of the Local Interstellar Cloud surrounding the heliosphere could be determined. They also discovered a so-called "Inner Source" of pickup ions. These ions are thought to originate from solar wind that is implanted in dust grains near the Sun. The particles are subsequently released as neutrals, and become ionized once again. Geiss' quest to measure and understand the composition of matter is not limited to the solar wind and pickup ions. He has also played a key role both in the in-situ measurements of molecular ions in comets and the interpretation of these data, and in the study of the composition of plasmas in the magnetospheres of Earth and Jupiter. On behalf of the Ulysses team, we wish Johannes Geiss "many happy returns" and many more scientific discoveries.
The outcome of the NASA Sun-Solar System Connections 2005 Senior Review held on 14-15 November was positive for Ulysses. The panel recommended continuation of the mission until March 2008 as proposed. This is reflected in the NASA budget allocation for the project. All spacecraft subsystems are operating nominally. On 1 June 2006, Ulysses will be at a radial distance of 3.85 AU from the Sun, and heliographic latitude 50º south of the solar equator.
Operations and Archive
All science operations during the reporting period have been nominal. The spacecraft team has performed a thorough review of the power and thermal situation on board the spacecraft, taking particular account of recent experience when platform temperatures reached their lowest values during the mission to date. Based on this, it has been decided to modify the current payload power-sharing plan. As a result, instruments that are not part of the "core payload" (and are presently turned off) will already be switched on again for short periods this year, prior to switch-off of the 25-watt Cold Case Heater in May 2007. The latter is needed to keep critical parts of the platform at a safe temperature when the spacecraft is far from the Sun.
Preparations, including the plans for ground-station coverage, are underway for the next nutation season that is predicted to start in mid-February next year. The nutation disturbance arises when the axial boom moves out of the shadow of the spacecraft body and is subject to asymmetric heating. As on previous occasions (in 1994/95 and 2000/2001), nutation will be controlled using the onboard CONSCAN system. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses.
After a journey of 12.5 years and more than two complete orbits the Sun, Ulysses is once again fully immersed in the fast solar wind from the coronal hole that covers the Sun's southern polar cap around solar minimum. This time around, the spacecraft had to climb higher in latitude, to almost 50ºS, before leaving the slower, more variable wind behind. In mid-1993, the transition occurred nearer to 35ºS. The explanation is related to the fact that the Sun's magnetic equator is still more highly inclined with respect to the rotational equator, even though the solar activity cycle is closer to its minimum, compared with the previous occasion. (At solar minimum, the Sun's rotational and magnetic axes are most closely aligned). Since the slower, more variable solar wind is mainly confined to the magnetic equator, this in turn means that, until recently, Ulysses continued to encounter slow wind once per solar rotation.
Observations from the solar wind instrument on Ulysses over the past year showed regular excursions from slow (~400 km/s) to fast (~700 km/s) wind every rotation. At the start of the year, however, the speed profile began to show a systematic increase in the minimum speed encountered with each subsequent solar rotation, and since early March, the speed has remained high. Given the fickle behaviour of the Sun, with large outbursts of activity occurring relatively late in the declining phase of the present cycle, it will be interesting to see whether Ulysses remains in the fast wind until its trajectory brings it to lower latitudes again after traversing the southern pole in February 2007.
All spacecraft subsystems are operating nominally. On 1 February 2006, Ulysses will be at a radial distance of 4.35 AU from the Sun, and heliographic latitude 40º south of the solar equator. As reported earlier, Ulysses was one of the missions reviewed by NASA's Sun-Solar System Connections Senior Review panel at its meeting on 14-15 November. The formal recommendations from the Review are expected to be made known early in this year.
Operations and Archive
All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. The current payload configuration will be maintained until spring 2007, when the spacecraft is close enough to the Sun to allow the Cold Case Heater to be switched off. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses.
A recurring theme in many of the results obtained by Ulysses is the unexpectedly large degree to which the heliospheric magnetic field deviates from the pattern expected from the combination of radial solar wind outflow and rotation of the foot point fixed in the solar photosphere (an Archimedean spiral). Existing models of systematic deviations from the spiral pattern require radial distances of several AU for a deviation of order 1 AU to develop. However, observations of "jets" of energetic electrons from Jupiter's magnetosphere, acquired by Ulysses during the 2003/4 distant encounter, show that such deviations are common within a radial interval of as little as 0.1 AU. Electron Jets were discovered during Ulysses' first Jupiter flyby in 1992, and were identified as brief (time scale of minutes to hours), highly anisotropic increases in intensity of MeV electrons flowing away from Jupiter along the heliospheric magnetic field. Jets were observed up to distances of the order of an AU from Jupiter, and were interpreted as evidence for direct magnetic connection to Jupiter's magnetosphere.
In the recent cases, the position of Ulysses relative to Jupiter was such that magnetic connection along the average spiral field could not have occurred, implying large deviations. If such large deviations are indeed common, they may play a significant role in the distribution of particles throughout the heliosphere by providing paths for particles to propagate parallel to the local field while crossing the average field. On average, the spiral heliospheric magnetic field still provides strong guidance to particle propagation, but individual field lines or flux tubes may deviate strongly over large distances normal to the average field. It is not yet clear how or why such large-scale deviations develop, whether they are consistently present throughout the solar cycle, or how to incorporate them into models of particle propagation.
A major milestone for Ulysses was reached on 6 October, namely the 15th anniversary of the launch from Cape Canaveral on board the space shuttle Discovery. After 15 years of operations, and a journey of 7 billion kilometres, the spacecraft and its scientific payload remain in good health, and no anomalies have occurred during the reporting period.
The situation concerning the budget for NASA's contribution to the mission, noted as a concern in the previous report, has shown significant improvement. The budget for mission operations and data analysis for Sun-Solar System Connections missions currently in their extended operational phase has largely been restored. Nevertheless, the outcome of the next Senior Review, to be held on 14-15 November, will still be important in determining the level of support available for Ulysses investigators funded by NASA.
On 1 November 2005, Ulysses will be at a radial distance of 4.7 AU from the Sun, and heliographic latitude 33º south of the solar equator.
Operations and Archive
All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. The current payload configuration will be maintained until Spring 2007, when the spacecraft is close enough to the Sun to allow the Cold Case Heater (needed to keep critical parts of the spacecraft platform at a safe temperature) to be switched off. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses.
In early September the Sun, although far into the declining phase of the current sunspot cycle, produced a display of major activity. This included one of the largest solar flares of cycle 23, an X17+ on 7 September, that occurred as the active region responsible rotated into view of the Earth on the Sun's east limb. A very large and very fast coronal mass ejection (CME) was also associated with this flare. At the time, Ulysses was positioned almost directly behind the Sun as seen from Earth, at 30 degrees south latitude and 4.8 AU from the Sun. This geometry provided Ulysses with a unique view of the source of the activity for several days prior to its appearance on the visible (from Earth) solar disk. Based on observations from the Ulysses radio experiment, it is likely that region 10808 produced at least 4 intense flares while on the far side as viewed from Earth. The X17+ flare produced an unusually intense radio burst observed by Ulysses, and the shock driven by the CME was observed in situ at Ulysses on 14 September, implying a transit velocity of ~ 1210 km/s over a distance of almost 5 AU! The radio bursts associated with some of the X-class flares occurring after the X17+ flare had surprisingly low intensities. This is consistent with what was seen for the 2003 "Halloween" events. A possible explanation is that the entire inner heliosphere was filled with energetic electrons to a flux level sufficient to block the plasma instability that would otherwise initiate the radio emission process.
The spacecraft and its scientific payload are in good health, and no anomalies have occurred during the reporting period. The situation concerning the budget for NASA's contribution to the mission remains a concern.
NASA has requested that the JPL Project formulate a Closeout Plan for Ulysses as a contingency measure in case funds are not restored. The project team at JPL has thus been instructed to draft a plan based on an assumed termination of routine spacecraft operations on 31 December 2005. The ESA Mission Operations Team members, located at JPL, have provided technical input where appropriate. This includes preparation of detailed end-of-mission procedures for spacecraft switch-off that is in any case required for the nominal end of mission in 2008.
On 1 May 2005, Ulysses will be at a radial distance of 5.1 AU from the Sun, and heliographic latitude 220 south of the solar equator.
Operations and Archive
All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. The proposed switchon of the Ulysses Gamma-ray Burst instrument to support post-launch calibrations of instruments on NASA's Swift gamma-ray burst satellite was deemed unnecessary based on the satisfactory in-orbit performance of the Swift payload. The Ulysses GRB instrument will therefore remain off until the on-board power/thermal situation improves (probably not before April 2007). The ESA Ulysses archive is accessible on-line at: http://www.cosmos.esa.int/web/ulysses.
The location of Jupiter with respect to the structure of the heliospheric magnetic field is precisely determined and non-central. This makes electrons emitted by Jupiter ideal tracers for studying particle propagation throughout the inner heliosphere. In mathematical terms, the propagation is usually characterized by a diffusion tensor. In recent work, Ulysses electron observations during the close (1992) and distant (2004) Jupiter encounters were analyzed to study the time dependence of the diffusion parameters. Since the observations from both periods were obtained during the declining phase of the solar cycle, it was expected that the electron intensities in 2004 would vary in the same way as the 1992 observations. This was found not to be the case, however. During the 2004 encounter, in contrast to 1992, Ulysses did not enter the Jovian magnetosphere but remained upstream of it. In mid-2002, the electron flux started increasing and displaying large short-term variations. These features lasted throughout the encounter, making the electron intensities less obviously correlated with the proximity to Jupiter compared with the first Jovian encounter. The suggestion is that the diffusion coefficients, and in particular perpendicular diffusion in the polar direction, are highly time-dependent.
The spacecraft and its scientific payload are in good health, and no anomalies have occurred during the reporting period. Nevertheless, because of the large distance from the Sun and the decreasing radioisotope thermoelectric generator (RTG) output, the general spacecraft thermal environment is currently well below the lower limits originally foreseen for the mission. The most critical item in this respect is the hydrazine fuel of the Reaction Control Subsystem, which must be prevented from freezing if at all possible. This places additional constraints on spacecraft operations, limiting the flexibility in assigning measurement opportunities to those experiments that are not part of the core payload. On a personal note, the Ulysses Mission Operations Manager (P. Beech, ESOC) retired at the end of 2004, after almost 20 years with the project.
Operations and Archive
All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. Data return has remained excellent (97% on average over the course of the mission, with an average of 98.2% during the last 5 years).
The unique perspective offered by Ulysses' orbit naturally lends itself to multi-spacecraft studies of transient solar wind features. Good examples of such studies are the so-called SOHO-Ulysses "quadrature" campaigns conducted when Ulysses is located off the limbs of the Sun as seen from Earth. Remote-sensing observations from SOHO can then be used to track disturbances leaving the Sun in the direction of Ulysses, and the in-situ measurements at Ulysses reveal the evolution of these structures as they travel outwards in the heliosphere. A prerequisite is the ability to identify the same parcel of plasma at both locations. The quadrature campaign in November 2002, at which time Ulysses was at 4.3 AU and 27º N off the west limb of the Sun, was particularly successful in this regard. For the first time, it was possible to identify the same very hot plasma remotely at the Sun with SOHO and in situ at Ulysses. Four large coronal mass ejections (CMEs) were observed by SOHO leaving the Sun in the general direction of Ulysses over a period of several days at the end of November. By the time they reached Ulysses some 15 days later, the interplanetary counterparts of these CMEs (ICMEs) had apparently merged to form a single large solar wind structure that drove a strong interplanetary shock. The plasma of this merged structure contained unusually large enhancements in highly ionised iron ions (charge state Fe16+), indicating a high-temperature source. Such high charge states are often seen in the solar wind and have been identified with ICMEs. The data from SOHO/UVCS also showed high Fe charge states, in particular in the aftermath of the 26 November CME. In this case, it was very hot plasma at 6-10 million degrees Kelvin that was apparently produced high in the solar atmosphere, above 1.5 solar radii. The most likely source of such hot plasma was a reconnection event occurring in post-flare loops.
The spacecraft and its scientific payload are in good health. Solar Conjunction operations began on 23 August, with the actual Conjunction (the ninth in the 14-year history of the mission) occurring on 1 September. At such times, the alignment of the spacecraft, Sun, and Earth is such that the radio communication ray-path passes close to the Sun, causing degradation to the signal. As a result of the poor link margins, the possible data rates were lower than usual, although actual data losses were kept to a minimum. Routine operations commenced once again on 8 September, with a return to normal data recovery.
Operations and Archive
All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. A core set of instruments providing the key science measurements is being operated continuously, together with one or more discretionary instruments depending on the thermal conditions and available power. In accordance with cost-saving measures agreed at the time of the 2003 Senior Review at NASA, as of 1 October 2004, the baseline telemetry coverage via the DSN will be reduced from 70 hrs per week to 35 hrs per week on average (although current planning through week 10 of 2005 shows a generally better performance). Normal coverage will resume in November 2006 to support the 3rd set of polar passes.
Following the interesting diversion provided by the Jupiter Distant Encounter earlier this year, the scientific focus of the mission has returned to the Sun and the evolution of the solar wind in the approach to activity minimum. As the spacecraft heads further south, it is expected to encounter relatively stable solar wind stream structures that co-rotate with the Sun, similar to those seen in 1992/93. A major difference, however, will be the polarity of the Sun's magnetic field that underwent a reversal during the recent solar maximum period. This in turn is expected to influence the paths of charged particles from the Sun, and cosmic rays, as they move through the heliosphere.
Another topic of continuing interest, and one to which Ulysses is making unique contributions, is the study of so-called "inner-source" pick-up ions. In contrast to pick-up ions of interstellar origin that are created through ionisation of neutral interstellar gas that penetrates the heliosphere, the precise origin of the inner-source ions remains uncertain. Among the possible candidates are neutral solar wind atoms that are implanted in, and subsequently released from, dust grains close to the Sun. The composition of the inner source is certainly not cometary in nature, making Sun-grazing comets, for example, an unlikely progenitor. Ulysses measurements have demonstrated clearly that the population of inner- source ions extends out as far as the orbit of Jupiter, suggesting that several different sources may be contributing. In addition to the intrinsic interest in these ions, they also form a potentially important seed population for injection into shock acceleration processes that give rise to fluxes of energetic particles of non-solar origin found throughout the heliosphere.
An important milestone in the mission was reached in February, when ESA's Science Programme Committee unanimously approved the funding to continue operating Ulysses until 31 March 2008. This latest extension, the third in the history of the mission, will enable Ulysses to acquire observations during a third set of polar passes. A key goal is to observe as fully as possible the influence of the recent polarity change in the Sun's magnetic field on the high-latitude heliosphere. The Jupiter Distant Encounter (JDE) campaign that commenced on 25 January was completed successfully on 8 March. The on-board tape recorder has been switched on again, marking the end of more than 40 days of 24-hr per day real-time coverage by the Deep Space Network. With the exception of the gamma-ray burst (GRB) instrument, the scientific payload was operated continuously during the JDE campaign without the need for power sharing. The spacecraft and scientific instruments are in good health. On 30 June, Ulysses will be at its maximum distance from the Sun (5.41 AU) heading south, having crossed the heliographic equator on 20 February.
Analysis of the data acquired during the Jupiter campaign is still in progress. Nevertheless, a number of interesting results have already emerged. The DUST instrument detected streams of dust particles flowing from Jupiter. First observed by the same instrument in 1992, the dust streams comprise grains no larger than smoke particles, and are believed to originate in the volcanoes of Jupiter's moon Io. The dust particles, which carry an electric charge, are strongly influenced by Jupiter's magnetic field. Electromagnetic forces propel the dust out of the Jovian system, into interplanetary space. The recent observations include the most distant dust stream ever recorded, at 3.3 AU from Jupiter. Another unusual feature is that the streams occurred with a period of about 28 days. This suggests that solar wind streams that co-rotate with the Sun play an important role. The most intense peaks showed fine structure not seen in 1992.
During its second encounter, Ulysses approached Jupiter from high northern latitudes, opening a window on previously unexplored parts of the Jovian magnetosphere. This was of particular interest to scientists studying Jupiter's natural radio emission, since a distinctive type of radio signal is believed to originate in the high-latitude auroral zones of Jupiter. These signals, which have a repetitive, burst-like character, have indeed been detected throughout the campaign period. The radio and plasma wave experiment on board Ulysses first detected bursts of radio waves occurring approximately every 40 minutes during the Jupiter fly-by in 1992. These so-called "Quasi-periodic", or QP-40 bursts were present for several hours, then faded away and reappeared a number of hours later. More recently, NASA's Chandra X-ray observatory detected similar QP-40 pulsations in X-rays emitted in hot spots in Jupiter's northern polar regions. Although not fully understood, these phenomena also seem to be triggered by streams of high-speed solar wind hitting Jupiter's magnetosphere.
The spacecraft and all scientific instruments are in good health. Preparations are underway for the Jupiter Distant Encounter campaign that will take place between the end of January and mid-March 2004. During this 50-day period, 24-hr per day real-time coverage by the Deep Space Network has been scheduled to allow the on-board tape recorders to be switched off. This in turn will permit the majority of the scientific payload to be operated continuously, without the need for power sharing. "Closest approach" to Jupiter occurs on 4 February, at a distance of 1684 Jupiter radii (~0.8 AU or 120 million km) from the planet.
On 1 February, Ulysses will be at a radial distance of 5.3 AU from the Sun, and will cross the heliographic equator heading south on 20 February.
Operations and Archive
All science operations during the reporting period have been nominal.
Even though the sunspot maximum of the current solar cycle (23) occurred in mid-2000, the Sun recently underwent a major surge in activity starting at the end of October. As noted in the report to SPC in February 2003, strong outbursts in the form of solar flares and coronal mass ejections (CMEs) are often seen during the declining phase of a solar cycle. The recent activity, however, was unusual both in its intensity, and its relative lateness. The largest solar flare of the series, rated at X28, occurred on 4 November while the responsible active region was on the Sun's west limb, rotating off the visible disk. Although quite far from the Sun (5.3 AU), Ulysses was well placed to observe the effects of this violent outburst, being more or less in the "line of fire". Analysis of data from the event is still underway, but indications are that the fast CME that was associated with the X28 flare swept over Ulysses, driving a significant interplanetary shock wave. Impressive enhancements in the flux of energetic particles, modulated by the passage of CME-related solar wind transients - and the passage of high-speed solar wind streams originating in a large, persistent, trans-equatorial coronal hole - were seen at Ulysses throughout the period of increased activity. The study of the precise mechanisms whereby solar energetic particles are distributed throughout large volumes of the heliosphere, and under which conditions efficient re-acceleration of these particles occurs, remains an important area of research using data from Ulysses. This unusual solar activity period appears to have been the Sun's final outburst before settling into a more stable configuration leading to the next solar minimum.
The mission is progressing nominally, with spacecraft and all scientific instruments in good health. One Anomaly Report was issued during the reporting period, concerning problems with the upload of new software for the GRU/DUST experiment. The problem was traced to the software code itself, and the previous version was subsequently re-loaded successfully pending corrections to the new code.
Regarding the possible extension of spacecraft operations beyond September 2004, positive recommendations have been given by the Solar System Working Group at its meeting on 23 April, and by the NASA Sun-Earth Connections (SEC) Senior Review Panel in its final report issued on 5 August. The SSWG in its recommendation noted that the scientific case for a further extension was strong; the Senior Review Panel recommended that Ulysses should be funded through its return to the north solar pole in 2008.
On 5 November, Ulysses will be at a radial distance of 5.23 AU from the Sun, and heliographic latitude 5.5º north of the solar equator, on its way to aphelion.
All science operations during the reporting period have been nominal.
Interstellar dust, which enters the heliosphere with the same velocity as neutral interstellar gas, was detected first by Ulysses. The incoming flux is expected to remain constant but, once inside the heliosphere, the dust particles are subject to forces exerted by the sun's gravitational field, solar radiation pressure and, since the dust is electrically charged, the heliospheric magnetic field (HMF). Gravity and radiation pressure affect the more massive grains, but the motion of low-mass interstellar dust is dominated by the magnetic force.
Theory indicates that changes in the HMF causes a solar cycle variation in the dust distribution. The dust is deflected toward higher latitudes (defocused) when the polar cap field is outward in the north hemisphere and the dust is deflected equator-ward (focused), when the north polarity is inward. The modulation also depends on the inclination of the HCS. Recent measurements by the DUST experiment on board Ulysses have confirmed that the flux does indeed change significantly with the solar cycle. In 1996, near minimum, the interstellar grains began a slow decrease that continued until near maximum (2000) when the flux levelled off and began to increase. The Ulysses DUST team has developed a model that includes changes in the field polarity and the inclination of the solar dipole, and which reproduces the observations and allows an estimate to made of the relative contribution of grains of different masses. The model predicts a steady increase in the flux of interstellar dust in the inner heliosphere to occur as a result of the recent reversal of the Sun's magnetic polarity.
The mission is progressing nominally, with spacecraft and all scientific instruments in good health. Unusually, three Anomaly Reports (ULY056, 57, and 58) were issued during the reporting period. The first concerned a command error caused by an incorrect bit-rate selection, while the second anomaly was the result of ground segment problems. The subject of Anomaly Report ULY058 ("EPC Switchover 2") is an autonomous switch-over from the prime to the redundant Travelling Wave Tube Amplifier (TWTA) and Power Conditioning units (EPC) that occurred on 15 February, and the subsequent failure to switch back to the prime units on 6 March. The redundant EPC/TWTA units are operating nominally, and no further attempts will be undertaken to switch back to the prime units at this time. A failure of the prime travelling wave tube is considered possible, since it has been in continuous operation for 12 years; however, the investigation into the anomaly is still in progress. The possible extension of spacecraft operations beyond September 2004 will be discussed by the Solar System Working Group at its meeting on 23 April. The Ulysses proposal to the NASA Sun-Earth Connections (SEC) Senior Review (to be held 10-13 June) will be submitted by the JPL Project to NASA HQ on 30 April. On 1 May, Ulysses will be at a radial distance of 4.84 AU from the Sun, and heliographic latitude 16º north of the solar equator, on its way to aphelion.
All science operations during the reporting period have been nominal.
For the first time since 1992, Jupiter is starting to figure strongly in the scientific activities of the Ulysses teams. On 5 February 2004, the spacecraft will encounter the giant planet for the second time. Unlike the 1992 fly-by, however, this will be a distant encounter (closest approach will be at 1684 Jovian radii from the planet's centre, compared with 6 Jovian radii in 1992). Another difference between the two fly-bys is that this time, the spacecraft will approach the planet from high northern latitudes. This difference is already apparent in the radio data from the URAP experiment on board Ulysses. In February and March 2003, URAP detected intense radio emission from Jupiter, at levels well above those seen in 1993 when Ulysses was at comparable distance from the planet (approx. 2.8 AU). This bodes well for the data to be acquired in November-December, when Ulysses will be much closer to the planet, and at 75º north of the planet's equator. Of particular interest are periodic auroral phenomena that produce both radio and X-ray bursts. For example, the Chandra X-ray Observatory recently observed quasi-periodic X-ray pulses, primarily from the northern hemisphere.
The mission, now in its 13th year of science operations, is progressing nominally, with spacecraft and all scientific instruments in good health. One Anomaly Report (ULY055 "Early termination of open-loop slew") was issued during the reporting period. The manoeuvre in question was the last in a series of four to calibrate the Upper Axial 1 (UA1) thruster. The early termination was traced to an error in a command file that had been edited by hand. Corrective action has been taken to ensure that in future, the Flight Dynamics Command Analyser software processes all command files that have been manually edited. The first three UA1 calibration manoeuvres were successfully executed, making it unnecessary to repeat the failed manoeuvre. As noted in the last report, preparations for the next NASA Sun-Earth Connections (SEC) Senior Review (to be held 10-13 June) are underway. The 2003 Review will address funding priorities within NASA for FY 2004-2005, and issue guidelines for FY 2006-2007. On 25 February, Ulysses will be at a radial distance of 4.65 AU from the Sun, and heliographic latitude 20º north of the solar equator, on its way to aphelion.
All science operations during the reporting period have been nominal. At its meeting in October, 2002, the Ulysses Science Working Team agreed unanimously to do away with the formal 1-year proprietary period for Ulysses data. Data is now placed in public archive immediately following verification by the PI teams. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses. Information regarding the data archive may be obtained from the Ulysses Data System Coordinator, Dr. C. Tranquille.
Ulysses continues to observe the transition from a transient-dominated to a quasi- stable heliosphere from its unique out-of-ecliptic vantage-point. As the level of solar activity decreases, the Sun's magnetic dipole has started to dominate the global magnetic field configuration once again. The angle between the dipole and rotational axes, nearly 90º at solar maximum, has also decreased. As a consequence, the heliospheric current sheet that separates inward (negative) magnetic fields from outward (positive) magnetic fields has become less inclined with respect to the solar equator. These large-scale changes have had a significant effect on the particles and fields measured at Ulysses. Since October 2002, there have been no excursions into the high-speed (~750 km/sec) solar wind from the northern polar coronal hole, and Ulysses has spent much of the time in more variable, lower-speed flows. This is also reflected in the energetic particle profiles, which have shown only modest increases, with very few transient-related events. Nevertheless, the Sun is known to produce some of its most powerful eruptions in the declining phase of the solar cycle, so things could change at short notice.
The mission, which recently completed its 12th year of science operations, is progressing nominally, with spacecraft and all scientific instruments in good health. On 4 November, Ulysses will be at a radial distance of 4.25 AU from the Sun, and heliographic latitude 28º north of the solar equator, on its way to aphelion.
All science operations during the reporting period have been nominal. In preparation for the next NASA Senior Review (mid-2003), and at the request of the Science Working Team, the Spacecraft Operations Team has conducted a preliminary study to assess the technical feasibility of continuing spacecraft operations beyond the currently approved end-of-mission (Sep 2004). The initial results of this study, which confirm the feasibility of extended spacecraft operations until 2008, will be presented to the SWT at its next meeting on 10-11 October.
Energetic particle measurements made by the COSPIN Low Energy Telescope (LET) are providing new insights into the seed populations of particles that are accelerated at both transient and co-rotating interplanetary shocks. From a study using composition data acquired by the LET during recent months, it has become clear that particles accelerated in co-rotating shocks (formed as a result of the interaction of stable fast and slow solar wind streams) form a distinct population, even in the presence of an elevated background of solar energetic particles from transient events. This study was possible owing to the combination of Ulysses' unique position - presently at the optimum distance and latitude for observing the in-situ effects of Corotating Interaction Regions (CIRs) - and the current (declining) phase of the solar cycle. The declining phase is characterised by both a return to more stable solar wind conditions that are conducive to the formation of CIRs, and the occurrence of large, transient, solar flare and CME events.
Observations from the solar wind plasma (SWOOPS) and ion composition (SWICS) instruments have been combined to carry out the first-ever study of solar wind flows from high-latitude coronal holes near solar maximum. This study has shown that, while the wind from coronal holes has unique chromospheric and coronal composition signatures, it can exhibit a range of flow speeds. High-speed wind (>700 km/s) can be produced in small as well as large coronal holes, although the highest speeds are found to originate in the centres of the largest holes. Ulysses has sampled the edges of coronal holes in detail in recent months as it returned slowly to lower latitudes. Here, the acceleration decreases and the freezing-in temperatures increase relatively smoothly into the surrounding solar wind, indicating a transition layer around the edges of coronal holes.
The mission is progressing nominally. At the end of May, Ulysses, which is now heading away from the Sun and back towards the ecliptic on the long leg of the orbit out to aphelion in 2004, will be 3.5 AU from the Sun, and 43º north of the solar equator. The spacecraft and scientific instruments remain in good health. Anomaly Report ULY053 was issued on 27 March following a problem with a software upload to the DUST instrument that occurred on 26 March. The memory readout that was performed to verify the upload showed an incorrect value in one memory location. After performing several unsuccessful attempts to reprogram the instrument, it was found that the new program code was overwriting a forbidden area of the memory. An updated reprogram sequence was successfully executed on 8 April, and the instrument has returned to full operational status.
With the exception of the anomaly referred to above, all science operations during the reporting period have been nominal. Because of the long lead-times involved in Deep Space Network scheduling, the SWT and spacecraft operations team have already started planning the distant encounter with Jupiter ("closest approach" distance 0.8 AU) that will occur in late 2003/early 2004. In particular, it is hoped to increase the power available to the scientific instruments during this period by limiting the use of the tape recorders, i.e., by maximising the real-time data coverage.
Ulysses continues to monitor the re-structuring of the extended solar corona at high latitudes following solar maximum. As reported earlier, the spacecraft spent more than 3 months at the end of 2001 fully immersed in the fast solar wind from the newly-formed north polar coronal hole. In recent weeks, however, regular excursions back into the slow wind have occurred. Indications are that Ulysses is presently skimming the boundary of the north polar hole, offering an unprecedented opportunity to study this boundary region in detail.
Ulysses has set the record for high-latitude observations of coronal mass ejections (CMEs). During the recent polar passes, CMEs were observed in situ at both 80º S and 80º N solar latitude.
Although Ulysses is still more than 4.5 AU from the planet Jupiter, the radio experiment has recently detected quasi-periodic (40-minute period on average) radio bursts from the gas giant. These bursts, which were also seen in 1992/93, are thought to be triggered by solar wind transients impacting Jupiter's magnetosphere. Similar periodicities have been observed in Jovian X-rays by the Chandra observatory, suggesting a common source mechanism.
The special operations to control spacecraft nutation were successfully terminated on 2 December, in line with predictions. As noted in the previous report, the spacecraft and ground operations teams are to be congratulated on the excellent job done during this demanding phase of the mission. The spacecraft and scientific instruments continue to function nominally. Ulysses is now heading away from the Sun and back towards the ecliptic on the long leg of the orbit out to aphelion in 2004. At the end of February the spacecraft will be 3 AU from the Sun, at heliographic latitude 55º north.
Ulysses has now completed its survey of the high latitude heliosphere under conditions of maximum solar activity, and has begun a new phase of its mission: the investigation of the global restructuring of the heliosphere following the polarity reversal of the Sun's magnetic field. We can summarise the key findings from the "solar maximum mission" as follows.
- The first exploration of the latitude dependence of solar wind characteristics (speed, temperature, composition) during maximum solar activity reveals an entirely different configuration of the 3-dimensional heliosphere compared to that observed during minimum solar activity. Although the solar magnetic field, corona and solar wind were highly variable in space and time, the magnetic field at Ulysses maintained a simple, dipole-like structure but with equivalent magnetic poles located at low latitudes rather than in the polar caps as at sunspot minimum. The spreading out of the field lines from these equatorial sources to high latitudes caused the solar wind to be deflected pole-ward.
- In contrast to solar minimum, where the intensities of cosmic rays showed systematic variations over the same latitude range, no measurable latitude variations were found at solar maximum.
- Solar energetic particles, most likely accelerated by shocks associated with coronal mass ejections, often reached the same intensity at Ulysses as measured simultaneously at Earth, regardless of the position of Ulysses. This implies that the particles filled the inner heliosphere almost uniformly, raising significant challenges for current models of acceleration and propagation of these particles.
- Ulysses has for the first time directly observed how the magnetic field of the Sun changes the amount of extra-solar material entering the solar system. The dust detector has counted an increasing number of interstellar grains since the beginning of 2000. This increase follows a depletion of interstellar grains observed by Ulysses after mid-1996 that was attributed to the deflection of the interstellar dust stream by the highly ordered heliospheric magnetic field. According to model calculations of the electromagnetic interaction of small interstellar dust grains with the heliospheric plasma, the return of interstellar dust to the solar system marks the beginning of large-scale disturbances of the heliospheric magnetic field during solar maximum. The model calculations also show that the configuration of the heliospheric magnetic field will cause the number of interstellar dust grains in the solar system to steadily increase until the next solar maximum.
Mission and Spacecraft Status
The mission continues to go well. By early December Ulysses will have reached the end of the 2nd North Polar Pass, being 70º north of the Sun's equator at a distance of 2.4 AU from the Sun. The highest latitude (80.2º N) was reached on 13 October. Data coverage continues to be excellent, and the spacecraft and scientific instruments remain in good condition. Procedures to control the spacecraft nutation have continued to be effective throughout the reporting period. Nutation operations are expected to end on 1 December when the thermal forcing drops below the previously established activation threshold. The spacecraft and ground operations teams are to be complemented for the excellent job done in controlling the nutation 24 hours per day over a period of many months.
The outcome of the 2001 NASA Sun-Earth Connection missions Senior Review, held on 10-13 July, 2001, was reasonably positive for Ulysses. As noted in a letter from E. Weiler, NASA's Associate Administrator for Space Science, to ESA's Director of the Scientific Programme, NASA has "approved funding for Ulysses operations until early 2004, which should provide data out to the region of the ecliptic during ramp-down of the current solar activity. We believe that this guidance is slightly inconsistent with current approved ESA funding for Ulysses. We believe that future dialog with you will lead to a common position on the cessation of operations."
After more than 4 years of exposure to variable, slow- to intermediate- speed solar wind conditions, Ulysses is now encountering fast wind from a large-scale polar coronal hole covering the Sun's north pole. Last year's passage over the southern polar regions was characterised by variable wind up to the highest latitudes; in contrast, the current high-latitude phase in the north shows signs of a return to more stable coronal conditions. In fact, the most recent high-latitude data are reminiscent of measurements made in 1995, near solar minimum. These results are not surprising given the fact that the maximum in solar activity has passed, and the Sun's magnetic field is starting to regain its simpler, dipole-like configuration. This change is accompanied by the switch in polarity associated with the 22-year magnetic cycle. Magnetic field measurements on board Ulysses echo this, showing a predominance of the new (negative) polarity in the latest data acquired at high northern latitudes.
The apparent transition to more stable solar conditions at high latitudes does not mean that solar activity levels are low in general. The Sun continues to flare and produce coronal mass ejections (CMEs), many of the latter being associated with increases in the flux of energetic particles seen at Ulysses (as discussed in previous reports). Such an event occurred in May, when a very dense CME (probably comprising filament material) swept over Ulysses. The interplanetary magnetic field ahead of the CME was highly compressed, and the associated shock wave generated intense fluxes of energetic particles. The event was particularly noteworthy, because at the time, Ulysses was close to the ecliptic at 90º to the Sun-Earth line (off the west limb of the Sun as seen from Earth). The CME was well observed by SOHO/LASCO, giving a good opportunity for interesting collaborative studies.
This and other events were among the topics discussed at a very successful joint Ulysses/ACE/Voyager Workshop held in Oxnard, California, on 15-19 October. A group of 70 investigators participating in on one or more of these key heliospheric physics missions came together to review recent results, initiate joint studies, and to take a critical look at the future of the field. It has become a tradition at meetings of this kind to examine current evidence regarding the location of the heliospheric termination shock. This is the boundary at which the solar wind becomes subsonic, and where we believe interstellar pick-up ions are accelerated to become the so-called anomalous cosmic ray component. The hope is that the two Voyager spacecraft, in particular Voyager 1 (now at a distance of 83 AU from the Sun), will soon cross the termination shock. Based on a variety of recent observations, the best estimates put the shock location at 90 ± 10 AU, making one or more shock crossings before 2005 increasingly likely. Many of these estimates rely on combined measurements from the network of heliospheric spacecraft that includes ACE and Ulysses.
Mission and Spacecraft Status
The mission continues to go well. Following the completion of its second visit to the Sun's south polar regions in mid-January 2001, Ulysses is now engaged in the phase of its orbit known as the "fast latitude scan". Passing through perihelion on 23 May, the spacecraft will cover 160º of latitude from maximum southern to maximum northern latitude in only 320 days. By the end of May, Ulysses will be 13º north of the Sun's equator at a distance of 1.34 AU from the Sun. Data coverage continues to be excellent, and the spacecraft and scientific instruments remain in excellent condition.
As noted in the previous report, spacecraft operations are currently complicated by the presence of nutation driven by asymmetric solar heating of the axial boom. The on- board Conscan system that is used to achieve active nutation damping requires a continuous uplink signal from the ground, and this in turn necessitated the temporary addition of Kourou (during February and March) to the network of DSN ground stations tracking Ulysses. Operations via Kourou went smoothly thanks to careful preparations by the ESOC support team. As expected, the rate of nutation build-up has increased in recent weeks, and is predicted to reach its maximum at the end of May. Nutation control procedures continue to be effective, however, and the science data is largely unaffected.
A comparison of heliospheric magnetic field data acquired at ACE in the ecliptic and at Ulysses has revealed that, even at solar maximum, the radial component of the field shows no latitude dependence. This result, which was one of the key findings from the first Ulysses polar passes in 1994/95, suggests that the same processes by which magnetic flux is redistributed close to the Sun at solar minimum continue to operate in the much more complex corona found at solar maximum.
As noted in earlier reports, a striking feature of the energetic particle data from the second south polar pass is the presence of transient increases in particle flux up to the highest latitudes. These increases are believed to be related to solar flares and coronal mass ejections occurring at lower latitudes. An even more surprising characteristic of these events is that, particularly during the decay phase, the intensities recorded at Ulysses over a wide range of energies are very similar to those found in the ecliptic. In many cases, the region of solar activity giving rise to particle events during Ulysses' south polar pass was found to be close to 20º north of the solar equator, implying transport of particles over more than 90 degrees in latitude. The exact mechanism that allows the charged particles to cross magnetic field lines with relative ease is not yet fully understood, although enhanced perpendicular diffusion is implied. Furthermore, since the duration of the events was typically 10 days or more, a significant range of longitudes was being sampled as the Sun rotated. Taken together, these findings tell us that solar flares and/or CMEs are able to fill the entire volume of the inner heliosphere with energetic particles in a short time, creating a "reservoir". Such wide-spread transport of particles was unimagined prior to Ulysses.
On-going studies of the high-latitude solar wind observed by Ulysses at solar maximum have revealed that there were fewer coronal mass ejections (CMEs) over the south pole than at mid-latitudes. This result is consistent with earlier coronagraph observations from the Solar Maximum Mission (SMM). Detailed comparisons with SOHO/LASCO data following the northern polar pass will further illuminate this behaviour.
Mission and Spacecraft Status
The mission, now in its eleventh year, is progressing well. Ulysses reached the maximum latitude (80.2º) of its second passage above the Sun's south polar regions on 27 November 2000, and formally completed the polar pass – the segment of the orbit above 70º solar latitude - on 16 January. Data coverage has been excellent in recent weeks, regularly exceeding 99%. This is partly a fortunate by-product of the nutation damping operations that commenced on 1 December 2000 (see below). The spacecraft and scientific instruments remain in excellent condition.
Since the beginning of December, spacecraft operations have been complicated by the gradual build-up of the nutation that disturbs Ulysses under certain conditions. First observed shortly after launch in 1990, the nutation is driven by asymmetric heating of the axial boom by the Sun as the spacecraft spins. For much of the mission, the axial boom is in the shadow of the spacecraft body, but during the high-latitude part of the orbit as Ulysses comes closer to the Sun, the Sun-spacecraft-Earth geometry is such that the boom becomes partially illuminated. The current period of nutation is the third such episode in the mission to date, the second one having taken place in 1994/95. Operational procedures developed prior to the 1994/95 build-up have proven highly effective in controlling the level of nutation to a point that scientific operations are generally unaffected. These procedures, employing the on-board Conscan system to achieve active nutation damping, require the presence of a continuous uplink signal from the ground. Because of this, data are transmitted in real time without the use of the on-board tape recorders. A necessary requirement for spacecraft safety, the 24 hr/day coverage also automatically increases the high bit-rate science data flow.
The JPL Project team has been informed that NASA plans to hold a so-called Senior Review of its Sun-Earth Connection missions (which include Ulysses and SOHO) on 17-20 July, 2001. Recommendations from the Senior Review will be used by NASA to give programmatic direction to the missions and programs concerned for 2002 and 2003; and to issue preliminary, tentative guidelines for 2004 and 2005 (to be reviewed in 2003). In this context, a recommendation concerning the funding of NASA's contribution to the extension of the Ulysses mission until September 2004 (ESA's contribution was approved by SPC in June 2000) will be made. All indications are that this recommendation will be positive.
As noted in the last report, recent findings from the mission featured prominently at the highly successful 34th ESLAB Symposium "The 3-D Heliosphere at Solar Maximum" held at ESTEC on 3-6 October in conjunction with the 10th launch anniversary celebrations. The proceedings of this symposium, to be published as a special issue of Space Science Reviews, are nearing completion and are scheduled to appear in late Spring/early Summer.
Now that Ulysses is once again heading back toward the equator following its third high-latitude excursion, an obvious question to be asked is: what did we see over the south pole at solar maximum? A preliminary answer would be: very similar conditions to those seen in the ecliptic. The solar wind has remained variable up to the highest latitudes, with no evidence of the persistent, high-speed flow found at solar minimum. In fact, the "high-speed streams" seen over the pole in 2000 very rarely exceeded 500 km/s, whereas the in-ecliptic wind frequently contained gusts of more than 600 km/s in the same time period. The variability of the high-latitude solar wind, probably a consequence of the absence of well-defined polar coronal holes at this time, is also reflected in the recent heliospheric magnetic field data recorded by Ulysses. Both field polarities continue to be present, with no clear evidence of the expected field reversal at the time of writing.
Continuing the trend that was started in July 2000 with the "Bastille day" solar event, the Sun has produced several major flares and Coronal Mass Ejections (CME) in recent months. A particularly large event occurred on 8/9 November. These solar storms generated significant fluxes of energetic particles, seen both in the ecliptic at 1 AU, and at Ulysses over the south pole. The exact mechanism whereby these particles gain relatively easy access to the high-latitude heliosphere remains an open question, one of the many that will be studied in the coming months.
Mission and Spacecraft Status
A major milestone in the history of Ulysses will be reached on 6 October, namely the 10th launch anniversary of the highly successful mission. The space probe will then be at 75 degrees south latitude, 2.6 astronomical units (386 million km) from the Sun, and will have covered a total of 4.8 billion km since launch. Maximum southern latitude (80.2 degrees) will be attained on 27 November. Data coverage during Ulysses return visit to the south polar regions of the Sun has continued at a consistently high level, regularly exceeding 98%. The spacecraft is in excellent shape, although a Disconnect Non-Essential Loads (DNEL) was triggered on 27 July, resulting in the switch-off of all experiments. As in the past, this DNEL (the tenth to have occurred during the 10-year operational lifetime of the spacecraft) was caused by a latching valve pulse in combination with a power peak at the time of a routine Earth-pointing manoeuvre. On this occasion, however, the valve pulses themselves were initiated as a result of anomalous sun sensor data.
Subsequent investigation revealed that the problem was due to a combination of so- called double sun-crossings of the X-beam sun sensor slits (a result of the small solar aspect angle), and an error in the documented threshold value for such double crossings. Until recently, a meridian sun sensor was operated in combination with the X-beam sensor to resolve possible ambiguities. Because the documented threshold for double crossings had not yet been reached, the meridian sensor had been switched off for power-saving reasons. Spacecraft recovery from the DNEL was nominal, and operational procedures have been modified to prevent similar occurrences in the future.
At its meeting on 6 June, the SPC approved the proposed extension of orbital operations until September 2004.
The latest results from the mission will feature prominently at the 34th ESLAB Symposium The 3-D Heliosphere at Solar Maximum to be held at ESTEC on 3-6 October in conjunction with the 10th launch anniversary celebrations.
One of the key questions currently being investigated using data from Ulysses is "Do the effects of low-latitude transient events on the Sun - solar flares, coronal mass ejections (CME), etc. - propagate to high solar latitudes?" The Sun was kind enough to present an important test case recently in the form of a large solar flare that occurred on July 14, 2000 (the Bastille day event). At approximately the same time as the peak in X-ray flux, the LASCO instruments on SOHO observed a bright, fast, halo CME directed toward the Earth. The CME produced the largest magnetic storm and aurora since the great storm of March 1989 that was responsible for the Hydro- Quebec power grid failure. Initial indications are that Ulysses, at 62 degrees south solar latitude (a separation of 80 degrees in latitude from the flare site), did indeed observe significant effects. A rapid increase in the flux of high-energy particles from the event, and a possible signature of the CME were seen. More details will be forthcoming as the full data sets become available.
Mission and Spacecraft Status
At the beginning of June, Ulysses will be at 57.5 degrees south latitude, at a distance of 3.4 astronomical units (507 million km) from the Sun. The spacecraft remains in excellent condition as it approaches the start of the next polar pass. Data acquisition has been at a consistently high level, regularly exceeding 98% coverage. The second visit to the Sun's southern polar regions officially commences on 8 September when Ulysses reaches 70 degrees latitude, and will last until 16 January 2001. The maximum southern latitude (80.2 degrees) is attained on 27 November. Although the nutation-like disturbance that affects the spacecraft as the axial boom receives progressively more illumination is not expected to appear before early December, preparations for dealing with this operational complication are already underway. These include readying the Kourou ground station that will be required to provide support in February/March 2001. A Mission Implementation Plan addressing these specific ESOC activities has been prepared. Given the useful body of experience gained during the 1994/95 nutation episode, the expectation is that no major problems resulting from nutation will occur, even though the solar forcing is ~1.5 times stronger than on the last occasion.
Given the outstanding success of the Ulysses mission to date, and the excellent prospects for new and exciting science in the future, ESA's Science Programme Committee unanimously approved the extension of orbital operations for a period of 2.75 years beyond the presently approved end-of-mission date of 31 December 2001, to September 2004. On the NASA side, funding is already approved for 2002, and a decision regarding a further extension will be taken in mid-2001.
The passage of Ulysses through the distant tail of comet C/1996 B2 (Hyakutake) on 1 May 1996 attracted significant media coverage when the two papers discussing the event were published in Nature on 6 April. In addition to their intrinsic scientific interest, the results presented by the magnetometer and solar wind ion composition experiment teams established a record for the longest comet tail ever observed.
At the time of writing, Ulysses continues to encounter variable solar wind conditions characterised by interactions between slow and fast streams. Unlike at solar minimum, the fast wind seen by Ulysses at moderately high latitudes can be traced back to isolated, mid- and low-latitude coronal holes. Fast wind from the polar coronal holes, if present at all, has not yet been observed.
Mission and Spacecraft Status
As might be expected, one of the key operational activities to have taken place since the last report was related to the transition to "Y2K" (the year 2000). Although none of the systems on board Ulysses have clocks that utilise the calendar date, the Y2K issue was certainly relevant to the ground segment. NASA's Deep Space Network introduced a series of operational measures designed to minimise any unforeseen effects of the roll-over from 1999 to 2000, including termination of support to all in-orbit spacecraft for a 30-minute period on either side of midnight, December 31. In the event, no serious problems occurred, although a minor Y2K-related software error in the command transfer system at JPL caused a slight delay in the return to nominal operations for Ulysses. The spacecraft and payload continue to function extremely well, with the start of the third polar pass now only a matter of months away. By the end of February, Ulysses will have reached 47 degrees south latitude, at a distance of 3.9 astronomical units (585 million km) from the Sun.
When comparing the current interplanetary conditions with those encountered by Ulysses at the same location more than six years ago, the effects of increased solar activity are evident. The stable solar wind structures that swept over the spacecraft once per solar rotation in 1993 have given way to a much more complex and less repetitive configuration. While there have been several "close encounters", Ulysses has not yet crossed the boundary into the fast solar wind flowing from the southern polar coronal hole. Given the rapid increase in sunspot number at the present time, it remains to be seen whether or not this will, in fact, occur at all during Ulysses' return to the polar regions.
One of the more controversial topics in recent years is the suggestion, based on sophisticated statistical time-series analysis of Ulysses energetic particle and solar wind data, that the structure of the interplanetary medium is not as chaotic as commonly supposed. Following up on their earlier work, in which the existence of solar g-mode oscillations in the interplanetary data was postulated, members of the Ulysses HI-SCALE team have now completed a thorough statistical analysis of Ulysses magnetic field data. This latest work apparently confirms the presence of numerous discrete modes in the data, and the scientists involved conclude that at least half of the energy in the interplanetary medium is in the form of such modes. These results are by no means universally accepted, however, and the physical mechanism by which such periodic variations, if real, are maintained remains unclear.
So-called solar type III radio bursts are a common feature of the natural radio emission present in the interplanetary medium, particularly at times of enhanced solar activity. The electromagnetic waves that make up the type III radio signals are known to be produced by streams of energetic electrons escaping from the Sun along interplanetary magnetic field lines. The electron beams are known to travel large distances, often reaching the orbit of Earth. A long-standing puzzle has been how the streaming electron beams are maintained over such large distances, since the waves they generate ought to scatter and disperse them quite rapidly. Ulysses electric field observations by the URAP team have now shown that the electron beams are stabilised by a non-linear process that "bunches" the particles and waves together in packets called "envelope solitons". This constitutes a major step forward in understanding the physics of this interesting phenomenon.
Many of the latest findings related to Ulysses' return to high- latitudes will be presented at the 34th ESLAB Symposium, to be held at ESTEC on 3-6 October, 2000. Looking even further ahead, a proposal in currently being prepared to extend the scientific operations of Ulysses beyond the current end-of-mission date (31 December, 2001), to 2004. There are compelling scientific arguments for such an extension, related in particular to the reversal of the Sun's magnetic field that will occur in the 2000-2001 time-frame. Furthermore, a detailed evaluation has shown that, despite the diminishing power levels on board, such an extension is technically feasible from a spacecraft systems point of view, and that sufficient flexibility exists in payload operations.
Mission and Spacecraft Status
By the beginning of November, Ulysses will be 38 degrees south of the Sun's equator at a heliocentric radial distance of 4.4 astronomical units. The spacecraft and scientific payload are in excellent condition. A spacecraft anomaly report was generated on 31 August following the detection of a missing format of science data in the playback data stream from the onboard Data Storage Unit (tape recorder) DSU2. While occasional missing data formats are not uncommon (so-called "data hits"), in this case there was no corresponding data gap in either the realtime or playback portions of the telemetry stream. Possible explanations are that either a single format was not recorded to the tape, or a single format was skipped when the tape recorder was reading out to telemetry. The missing data format coincided with a switch from one tape track to another, with a corresponding reversal in the direction of tape travel. A total of approximately 4000 such track switches have taken place on the two tape recorders since launch, but this is the first time an anomaly of this kind has been detected. Both tape recorders will be monitored closely to see if the behaviour is repeated in the future. The impact of the data loss resulting from the present anomaly is minimal, and the level of data return (98% on average) has continued to meet the high standard established in the course of the mission.
Although quite variable, solar activity continues its upward trend, and the interplanetary weather throughout the inner heliosphere reflects this fact. A comparative study carried out using data from Ulysses, SOHO and the Wind spacecraft, has shown that the characteristic signatures of transient increases in the flux of energetic particles recorded at 1 AU, evolve significantly as they propagate out to the location of Ulysses. While it is often possible at 1 AU to associate specific increases in particle flux with specific events occurring at the sun (usually coronal mass ejections or solar flares), this is rarely the case at 4-5 AU. By the time the particles arrive at Ulysses, different populations may have merged, and may even have undergone significant reacceleration at the large-scale shock fronts that travel out from the Sun. Although not as prominent as at solar minimum, corotating interaction regions (CIRs) are still an important part of the solar wind structure, and may also act as mixing and accelerating agents for the particles. Under these conditions, the particles can no longer be thought of as tracers of the large-scale structure of the heliosphere, since local effects dominate the picture. A key objective of the mission will be to observe the evolution of these particle populations as the spacecraft climbs to higher and higher latitudes.
Charge exchange with solar wind protons is the primary ionization process for interstellar hydrogen atoms traveling through the heliosphere. Ulysses solar wind data have been used to examine and quantify variations in charge exchange, which has implications for, for example, the interpretation of observations of scattered Lyman-alpha radiation. It is found that the charge exchange rate is higher by an order of magnitude in corotating interaction regions, and the compression regions ahead of fast coronal mass ejections, than in non-compressed solar wind. As a consequence, much of the interstellar hydrogen at low latitudes is ionized in pulses, as the compression regions sweep past. Ulysses observations have revealed that the charge exchange rate is higher at low than at high latitudes, and that this rate drops off more slowly than the inverse square of heliocentric distance. The latter result is attributed to the growth of the CIRs as they propagate outwards through the heliosphere. Since the pick-up ions produced by the charge-exchange process form the seed population for the so-called anomalous cosmic rays, these results have far-reaching consequences. As in many cases, Ulysses data are being used to infer not only the local conditions in the solar wind, but also the global structure of the heliosphere.
Ulysses magnetic field data acquired during the first two polar passes have been used recently to infer global properties of the Sun's coronal magnetic field extending back in time to the mid-19th century. Relying on the excellent correlation found between the so-called "aa" index (a measure of the variability of the geomagnetic field) and a coupling function that includes the radial component of the heliospheric magnetic field, an historical record of the coronal magnetic field has been constructed going back as far as 1868. A critical part of the calculation relies on the Ulysses finding that the radial component of the heliospheric field is independent of solar latitude. A particularly striking result is the fact that the inferred coronal field has increased by a factor 2.3 in the past 100 years, perhaps as a result of chaotic changes in the solar dynamo. Although not well understood, a connection is believed to exist between the Sun's magnetic field and its luminosity, indicating possible implications for the global climate of the Earth.
Mission and Spacecraft Status
By the end of May, Ulysses will be nearly 30 degrees south of the Sun's equator at a heliocentric radial distance of 4.9 astronomical units. Spacecraft operations have proceeded smoothly during the reporting period, the only exceptions being a Disconnect Non-Essential Loads (DNEL) event on 15 February, and 16% underperformance of the nominal Lower Axial (LA1) thruster while executing a routine manoeuvre on 4 February. The DNEL was the ninth such event to have occurred in the 8.5 years of operations, and recovery was nominal. As in the past, the DNEL was most likely triggered by a random Main Switch current spike occurring in coincidence with the pulse associated with the LATCHING VALVE ON command at the start of the planned manoeuvre. Thruster underperformance during a manoeuvre has occurred sporadically since mid-1996, and is attributed to the presence of small amounts of gas in the monopropellant hydrazine (to be expected given the age of the fuel). Neither of these anomalies indicates any degradation in the corresponding spacecraft systems. The level of data return has continued to meet the high standard established in the course of the mission, and has generally exceeded 95%.
The onset of cosmic-ray modulation, one of the fundamental processes related to the start of the new sunspot cycle, is clearly evident in the recent data from Ulysses. The precise events that triggered the observed decreases in cosmic ray flux have so far, however, remained elusive. Preliminary discussions at the Aphelion Workshop last year were followed up by more detailed analysis at a second workshop in March that was organised by the COSPIN experiment team. While periods of enhanced solar activity and the related transient solar wind disturbances (generally assumed to be one of the key factors in preventing the incoming cosmic rays from reaching the inner heliosphere) could be identified, their timing and/or extent does not appear to match the cosmic ray signatures. Possible alternatives, including large-scale reorganisations of the Sun's magnetic field, are being investigated.
The effects of the general increase in solar activity associated with the new cycle are also apparent in the magnetic field data from Ulysses. Both the strength of the magnetic field, and the number of interplanetary shock waves, have been observed to increase in concert with the number of sunspots recorded on the surface of the Sun. These trends, while not spectacular as yet, clearly point to the more disturbed conditions in the heliosphere that Ulysses will encounter in the next two to three years.
Measurements of the mass distribution of local interstellar dust particles by the Ulysses DUST experiment are revealing interesting deviations from models based on a sublimation-condensation equilibrium with the gas phase of the local interstellar medium. The data show an excess of large particles (radii greater than 0.3 microns) compared with expectations. According to the DUST team, the Ulysses results imply either a direct stellar source for these particles, or the possibility that large grains survive erosion by high-speed gas that has been shocked by supernovae.
Mission and Spacecraft Status
Ulysses has continued to carry out its mission in a highly effective way during the reporting period. Retracing the path it first followed a little over 6 years ago, the spacecraft is now 20 degrees south of the Sun's equator on its way to the second south polar passage at the end of 2000. Data recovery has been generally good, although a minor shortfall was experienced at the end of December when Ulysses released scheduled tracking time on the Canberra DSN 34-metre antenna to support SOHO operations following their Emergency Sun Reacquisition (ESR). All spacecraft subsystems and experiments continue to perform well.
The continued high level of scientific interest and productivity within the Ulysses investigator teams was amply demonstrated at the recent Aphelion Workshop, held at the end of October in Oxnard, California. Fifty scientists participated in the meeting, including representatives from the SOHO, ACE and WIND communities. The workshop was organised around four themes: Data comparisons at 1 and 5 AU; Science goals for solar maximum; Shocks, waves, particles and turbulence; Interstellar/astrophysical aspects of Ulysses. Separate Working Groups, lead by one or more members of the Ulysses science teams, were established in advance of the workshop for each of the themes, and participants were encouraged to provide input to the Working Groups prior to the meeting. This was facilitated by the creation of dedicated Workshop Web pages on the Ulysses/ESA Web site, which will continue to be used to provide progress reports on activities that were initiated as a result of the workshop. Although no formal proceedings of the workshop will be produced, it is anticipated that the highly successful meeting will result in a large number of papers appearing in the literature.
As expected, the Working Group "Data comparisons at 1 and 5 AU" attracted a large number of participants, with very active discussions focusing on the recent solar events that were observed at both 1 AU (ACE/SOHO) and at the location of Ulysses near the orbit of Jupiter. A major goal was to determine the dominant physical processes that are responsible for the evolution of energetic particle fluxes and solar wind structures with radial distance from the Sun. Important progress was made in understanding the solar wind features, but several questions remained with regard to the energetic particles. A comparison of solar wind elemental and charge-state composition during the period of ACE-Ulysses radial alignment showed very little correlation between the two spacecraft, suggesting that the bulk of the solar wind flow is made up of small-scale "blobs" or mixtures of transient and stream-related plasma rather than coherent, large-scale structures.
In addition to investigations into the global structure of the heliosphere, which form the core of the mission, Ulysses has made, and continues to make, fundamental contributions in a broader astrophysical context. A good example is the recent detection by the Ulysses GRB experiment of the gamma-ray burst from the soft gamma repeater SGR 1900+14 on 27 August, 1998. This was the largest gamma burst recorded to date, and the Ulysses GRB data provided unique information on the peak flux from this newly-discovered "magnetar", and the rapid oscillations observed during its decay.
The ESA Ulysses archive is accessible via the World Wide Web at http://www.cosmos.esa.int/web/ulysses. A boxed set of CD-ROMs containing the complete data archive covering the period from launch to October 1995 (including the Jupiter Encounter) is in preparation and will appear as ESA publication SP-1230. Further information regarding the Ulysses data archive may be obtained from the Ulysses Data System Coordinator, Dr. C. Tranquille.
Mission and Spacecraft Status
The Ulysses spacecraft, heading progressively further south of the Sun's equator, attained its maximum distance from the Earth (6.35 astronomical units, or 951 million km) at the end of August. This also marked the sixth Conjunction period of the mission, when the Earth, Sun and Ulysses are in close alignment. Because the Sun-Ulysses-Earth angle becomes small (0.9 degrees during this particular Conjunction), careful manoeuvring of the spacecraft was required to keep the Solar Aspect Angle (the angle between the spin axis and the spacecraft-Sun vector) within predefined operational limits. Data recovery during the Conjunction period has been somewhat reduced, both because of expected loss in performance of the downlink as a result of the proximity of the line-of-sight to the Sun, but also due to non-availability of ground stations caused by the SOHO recovery operations. All spacecraft subsystems and experiments continue to perform well.
The surge of solar activity at the end of 1997 reported in the previous Science Highlights was followed by a more extended outburst in April and May of this year. As in the earlier case, the effects were clearly observed both at 1 AU and at the location of Ulysses, five times further away. A comparison of data obtained by the instruments on board Ulysses and data acquired at 1 AU will form one of the themes of the Ulysses Aphelion Workshop, to be held at the end of October in Oxnard, California. Other topics will include a general discussion of the scientific questions to be addressed by Ulysses during the solar maximum phase of the mission, and a detailed examination of new insights gained into shock physics as a result of Ulysses observations.
As in past years, Ulysses results featured prominently at the recent COSPAR Scientific Assembly. Highlights included further developments of the heliospheric magnetic field model first proposed by Fisk to explain the persistence of recurrent increases in the flux of energetic particles at high heliographic latitudes observed by Ulysses. Recent work has shown that magnetic reconnection in the streamer belt at low latitudes, a necessary element of the Fisk model, could provide a natural way to open up closed magnetic field loops, thereby allowing loop material to flow out into the heliosphere as the slow solar wind.
Ulysses' slow transit of the equatorial regions at a radial distance of 5 AU around aphelion, coupled with the very quiet solar conditions existing during much of 1997, provided a unique opportunity to measure the true radial variation of the anomalous cosmic ray (ACR) component in the inner heliosphere, free from latitudinal effects. ACRs are interstellar neutrals that become ionized in the solar wind, and subsequently gain energy in the distant heliosphere to become part of the cosmic ray flux. This measurement, made in collaboration with SOHO and of importance for cosmic ray transport theories, was not possible during the in-ecliptic phase of the mission (prior to Jupiter encounter) because of the high level of solar activity that masked the ACR component.
The ESA Ulysses archive is accessible via the World Wide Web at http://www.cosmos.esa.int/web/ulysses. The data from the Ulysses Jupiter Encounter (February 1992) have been archived by the NASA Planetary Data System, and have recently been issued on CD-ROM. These data, together with a set of CD-ROMs containing the prime mission archival data are available from the Ulysses Data System Coordinator, Dr. C. Tranquille.
Mission and Spacecraft Status
On 17 April, after travelling for more than seven years and covering 3.8 billion kilometres, Ulysses completed its first orbit of the Sun. All spacecraft subsystems and experiments continue to perform extremely well, and the recent MIDAS (Multi-project Investigation During Alignment of Spacecraft) campaign involving Ulysses, SOHO, ACE and Wind was very successful in obtaining near-continuous coverage at high bitrate over a period of several weeks. Moving slowly southward on its post-aphelion trajectory, Ulysses will cross the ecliptic in mid-May at a distance of 5.4 astronomical units from the Sun.
After an extended period of quiet solar conditions, two major solar flares occurred in quick succession at the beginning of November. These events, which produced significant interplanetary disturbances, have been the subject of a number of comparative studies involving Ulysses and the fleet of spacecraft at 1 AU, including SOHO. Although the response to the solar activity, both in the form of energetic particles arriving promptly from the Sun, and the more slowly propagating magnetic disturbances, followed relatively well-established patterns at the Earth, the signatures at Ulysses, which was separated in both radial distance and longitude from Earth, were more complex. By studying these effects in detail, it is hoped to gain a better understanding of the global structure of the disturbances and their influence on particle transport. Such multi-spacecraft studies will form an increasingly important role in the analysis of Ulysses data as the solar cycle develops.
Another topic addressed by Ulysses relates to the much larger-scale issue of cosmic ray confinement. By measuring the isotopic composition of cosmic ray aluminium and chlorine with high precision, Professor John Simpson and his co-workers on the COSPIN experiment have derived a cosmic ray "age" of 20 Myrs, requiring that the cosmic ray particles spend a significant fraction of their lifetime in the low-density galactic halo.
These and many other findings were reported at the recently held Ulysses Science Working Team and European Geophysical Society meetings.
The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses. A set of CD-ROMs containing the prime mission archival data are now available from the Ulysses Data System Coordinator, Dr. C. Tranquille.
Mission and Spacecraft Status
Ulysses crossed the heliographic equator in mid-December, and is now heading slowly southwards at a distance of 5.4 astronomical units from the Sun. At the end of February, the Sun, the Earth and Ulysses will be radially aligned, providing an excellent opportunity to study the spatial evolution of interplanetary disturbances and other transient phenomena. The spacecraft and scientific payload have operated without problem during the reporting period, and the data return has been consistently high.
As noted in the previous report, Ulysses is now largely immersed in slow solar wind with a typical speed of 350-400 km/s. The magnetic field polarity observed at Ulysses has remained predominantly positive (characteristic of the northern solar hemisphere), even at very low northern latitudes. This implies that, consistent with the low level of solar activity at the present time, the inclination with respect to the solar equator of the heliospheric current sheet is very small, and that the current sheet itself is rather flat. A puzzle remaining from the "fast latitude scan" (the rapid south-to-north transit made by Ulysses in 1994/95) has recently been solved. It was noted at that time that cosmic ray observations made in the southern and northern hemispheres showed a clear asymmetry, indicating that the heliospheric current sheet was offset to the south by some 5-10 degrees. This could only come about if the average radial magnetic field in the south was somewhat greater than in the north, which appeared not to be the case. Recent analysis including in-ecliptic data has shown, however, that the southern field at the time of the fast latitude scan was indeed larger by an amount sufficient to account for the cosmic ray data.
The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses. The peer review process for the CD-ROMs containing the prime mission archive has been carried out, and the first formal issue is planned for March 1998.
Mission and Spacecraft Status
Spacecraft operations have been nominal throughout the reporting period, and the spacecraft subsystems and payload remain in excellent health. The level of data coverage has been consistently high, in excess of 97% on average. At the end of October, the spacecraft was 2 degrees north of the solar equator at a distance of 5.3 astronomical units from the Sun.
Since mid-June, as Ulysses continued its gradual descent to aphelion, slow solar wind streams with typical speeds of 350-400 km/s have dominated interplanetary conditions at the location of the spacecraft. Recurrent features related to solar rotation, characteristic of intermediate latitudes, have been largely absent in the recent data. With solar activity starting to pick up again, the scientific focus of the mission is beginning to shift towards transient phenomena. Particularly important in this regard are the excellent opportunities currently available for collaborative studies with SOHO and other members of the solar/heliospheric fleet of spacecraft now in orbit.
Ulysses has featured prominently at a number of scientific meetings during the reporting period, including the 31st ESLAB Symposium, IAGA, and the 25th International Cosmic Ray Conference. Regarding Ulysses' contribution to cosmic ray studies, one of the most intriguing findings is related to the Ulysses measurements of the cosmic ray electron flux, which continues to display a different temporal evolution to that of protons. Given the fact that the two species are of opposite charge, these differences lend support to models of cosmic ray modulation in which charge-dependent drift effects in the large-scale heliospheric field are important at solar minimum. The effect on charged-particle propagation of systematic latitudinal transport of heliospheric magnetic field lines (proposed by Fisk in 1996) remains another important theme.
The ESA Ulysses archive is accessible via the World Wide Web at URL: http://www.cosmos.esa.int/web/ulysses. Recent submissions include solar wind data extending through mid-1996.
Mission and Spacecraft Status
With the exception of a Disconnect Non-Essential Loads (DNEL) event which occurred on April 1, spacecraft operations have been nominal since the last report. As on past occasions, recovery from the DNEL was accomplished without problems, and the spacecraft subsystems and payload remain in excellent health. The level of data coverage has been consistently high, in excess of 95% on average. At the end of May, the spacecraft was 10 degrees north of the solar equator at a distance of 5.1 astronomical units from the Sun.
As discussed in the previous report, Ulysses has now returned to the region of the heliosphere close to the Sun's equatorial plane where slow solar wind from the streamer belt interacts with faster wind from the poles. Not surprisingly, investigations in recent months have focused on these interactions, as well as effects related to the heliospheric current sheet (the boundary separating the opposed magnetic fields originating in the north and south polar coronal holes). Although not equipped with imaging instruments, Ulysses has also participated in observations of comet Hale-Bopp. Ulysses investigators, together with members of the "The Ulysses Comet Watch" team (an international group of amateur astronomers), have used solar wind data from the spacecraft to study changes in the comet's plasma tail. Other highlights include significant improvements in the measurement of cosmic-ray iron and nickel isotopes. The measured isotopic ratios, which are generally solar system-like and not neutron-rich, suggest that cosmic ray acceleration occurs in large-scale shocks in the interstellar medium, rather than by explosive nucleosynthesis.
The ESA Ulysses archive, accessible via the World Wide Web, contains data sets covering the whole of the prime mission (up to October 1995). It is planned to release the prime mission data on CD-ROM in the course of the year.
Mission and Spacecraft Status
By the middle of February, the latitude of Ulysses had decreased to 16 degrees north of the solar equator at a distance of nearly 5 AU from the Sun. The mission continues to go well, with all spacecraft subsystems and scientific instruments functioning nominally. No anomalies occurred during the reporting period, and the level of data coverage remained in excess of 95% on average. It should be noted that the excellent data return achieved throughout the mission to date, a key factor in the high-quality scientific output of Ulysses, is due in large part to the dedication and enthusiasm of the Mission Operations and Ground Segment teams.
At the beginning of September, following an extended period of nearly 18 months during which Ulysses had been immersed in fast-flowing solar wind from the north polar coronal hole, the spacecraft's latitude had decreased to the point where streams of slower wind from the streamer belt began to be encountered once per solar rotation. Since then, all the features usually associated with well-developed corotating interaction regions, such as forward and reverse shocks, and recurrent increases in the intensity of energetic charged particles (together with correlated periodic decreases in the cosmic ray flux) have been observed. The current phase of the Ulysses orbit, as the spacecraft approaches aphelion, is characterised by relatively slow changes in latitude and radial distance. From this quasi-stationary, low-latitude vantage point, Ulysses is well-placed to monitor transient solar wind features such as Coronal Mass Ejections (CMEs) that have moved out from the Sun, past the orbit of Earth. A number of CME events are being studied in collaboration with other spacecraft (e.g., SOHO and WIND), including the large CME that passed the Earth in mid-January.
Guest Investigator Programme
All nine proposals for Guest Investigations that were received in response to the ESA AO for the Ulysses Guest Investigator Programme have been recommended for selection. Provided that appropriate funding can be obtained, the Guest Investigators will formally join the Ulysses science team at the next Science Working Team meeting, to be held in ESTEC in April.
Mission and Spacecraft Status
Continuing its slow equatorward descent in the northern hemisphere, Ulysses will have reached a latitude of 20 degrees north of the solar equator by the end of November, at a distance of 4.6 astronomical units from the Sun. The mission, now in its seventh year, continues to go well, with all spacecraft subsystems and scientific instruments functioning nominally. The only anomalies to have occurred during the reporting period were a Disconnect Non-Essential Loads (DNEL) event in August, and two instances of thruster under-performance as a result of gas bubbles in the hydrazine. Recovery from the DNEL was nominal, and the formation of small amounts of gas in the propellant is not considered abnormal given the age of the system.
A comprehensive collection of papers describing the results obtained during the pole-to-pole transit and northern polar passage is to appear in December as a special issue of Astronomy and Astrophysics. An important new finding in recent months concerns the first-ever measurement of the isotope helium-3 in the interstellar gas. Detected at high latitudes by the SWICS instrument in the form of pick-up ions, interstellar helium-3 provides clues to the amount of dark matter produced in the Big Bang. The Ulysses results, which show a surprisingly small increase in the amount of the helium isotope since the formation of the solar system, can only be explained if the amount of dark matter in the early universe was greater than previously thought.
Insights gained from the unique Ulysses data set are enabling important progress to be made towards one of the primary scientific goals of the mission, understanding cosmic ray modulation. This was evident at a recent ISSI Workshop on the topic, in which theorists and experimenters in the field participated. Once again, an important theme was the large-scale latitudinal motion of heliospheric magnetic field lines.
Guest Investigator Programme
Nine proposals have been received in response to the ESA AO for the Ulysses Guest Investigator Programme. A review process has been established, and it is expected that, contingent on their obtaining appropriate funding, successful GIs will join the Ulysses science team early in 1997.
Mission and Spacecraft Status
An important milestone was reached at the end of March, namely the 2000th day of highly successful Ulysses mission operations. All spacecraft subsystems and scientific instruments continue to perform well, and the combined spacecraft and ground segment teams have maintained their impressive record in providing consistently better than 95% data coverage. As of 1 June, Ulysses will be almost 4 astronomical units from the Sun, 35 degrees north of the solar equator.
Results from Ulysses featured prominently at a Workshop on "Corotating Interaction Regions (CIRs)" that was held at Schloss Elmau, Germany, from 11-15 March. Of particulat interest in this context are the global periodic variations seen in the energetic particle and cosmic ray measurements made by Ulysses. These phenomena are believed to be associated with CIRs, which are solar wind structures that form predominantly at low latitudes as a consequence of interactions between fast and slow solar wind streams. The question raised by the Ulysses measurements is how do the effects of CIRs operate at high latitudes, where the CIRs themselves are no longer observed? Recent work on the large-scale latitudinal motion of heliospheric magnetic field lines, demonstrating the possibility of magnetically connecting the polar regions to low-latitude CIRs, may provide the answer.
Mission and Spacecraft Status
By the end of February, the latitude of Ulysses will have decreased to 45 degrees north of the solar equator as the spacecraft heads away from the Sun towards aphelion. The mission, now in its sixth year, continues to go well, with all spacecraft subsystems and scientific instruments functioning nominally. The only anomaly to have occurred during the reporting period was a Disconnect Non-Essential Loads (DNEL) event, the sixth such occurrence in the history of the mission. Recovery from the DNEL was nominal.
As stated in the previous report, observations during the northern polar pass revealed features generally similar to those found in the south. However, several experiments have recently reported small asymmetries in the data. For example, the solar wind speed is 2 percent higher on average in the north as compared with the south, while the density is lower in the north by 8 percent. Taken together, these measurements indicate a 6 percent lower mass flux in the north compared with the south. The cosmic ray data also show an asymmetry, with 10 percent higher fluxes measured over the north pole than over the south. Curiously, the observed magnetic field strength shows no latitudinal differences. Overall, the Ulysses data suggest that the solar corona exhibited a north-south asymmetry of roughly 10 degrees with respect to the solar equator during the pole-to-pole transit.
Mission and Spacecraft Status
On 30 September, five years after launch, Ulysses completed the first phase of its highly successful exploratory mission to study the Sun's environment from the unique perspective of a solar polar orbit. Between 19 June and 29 September, one year after its historic south polar pass, the spacecraft flew over the Sun's northern polar regions, reaching a maximum latitude of 80.2 degrees north of the equator on 31 July. The mission continues to go well, with all spacecraft subsystems and scientific instruments functioning nominally.
In recent months, as expected, the joint ESOC-JPL Mission Operations Team located at JPL had to contend with a renewed build-up of the nutation disturbance that, if unchecked, would cause the spacecraft to oscillate about its spin axis. Following the procedures developed during the previous two periods when nutation was present, the onboard Conscan system was operated in closed-loop mode in order to keep the spacecraft's high gain antenna pointed at the Earth. In line with predictions, the nutation, which peaked in early May, had decayed by early September. The cooperation of NASA's Deep Space Network in providing the round-the-clock tracking support needed to operate Conscan throughout this period is gratefully acknowledged. Nutation is not expected to return until January 2001.
In retrospect, the first five years of the Ulysses mission have covered many ground-breaking activities from an operational standpoint. The launch in October 1990 was the first occasion that ESA staff were operationally responsible for a Shuttle payload during an eventful six hour period which culminated in successful deployment and the start of the journey to Jupiter. The unexpected onset of nutation shortly after deployment of the axial boom presented a further challenge in analysis and understanding of the mechanisms at work on board the spacecraft causing the anomaly. Correct interpretation enabled the preparation for the return of nutation in 1994 to be carried out in an orderly manner followed by the successful control of nutation during a period of over one year. These activities stretched to the limit the NASA and ESA ground facilities used for the task as well as the operational staff located at JPL and at the ground stations. The fact that the acquired scientific data were not degraded is proof of success of these operations.
The encounter with Jupiter in February 1992 was also a first for ESA operational staff and presented many challenges. A unique method of conducting the operations was devised which provided the experiment teams with the capability to access their experiments in near real time during the actual encounter. Looking ahead, science data gathering activities will continue throughout the entire second solar orbit, no doubt leading to new operational challenges.
Following its rapid passage through the equatorial region dominated by slow solar wind, Ulysses is now once again immersed in fast wind, this time from the northern polar regions. The spacecraft crossed the boundary separating the predominantly slow wind (average speed ~400 km/s) from the fast wind (average speed ~750 km/s) at a solar latitude of approximately 20 degrees north. Perhaps not surprisingly, the data from the northern hemisphere obtained to date reveal features that are generally similar to those found south of the equator. As the detailed analysis progresses, however, subtle differences will undoubtedly be uncovered.
Rather than present a detailed discussion of the latest findings from the northern polar pass, many of which will be presented for the first time at a Workshop to be held in California at the end of October, this report focuses on the key results from the Ulysses prime mission as a whole. Highlights from the out-of-ecliptic, in-ecliptic and Jupiter fly-by parts of the mission are listed. The list is by necessity not exhaustive; nevertheless, it serves to illustrate the richness and diversity of the scientific harvest from the mission to date. Almost without exception, the most important results have come from the analysis and interpretation of combined data sets from different instruments, a natural consequence of the high degree of integration and collaboration within the Ulysses science team.
- Characterisation of two fundamentally distinct solar wind regimes with common, sharply delineated chromospheric and coronal boundaries: slow wind from the streamer belt, fast wind from the (polar) coronal holes.
- No concentration of magnetic flux at the poles in the heliospheric magnetic field, implying that the dipole-like configuration of the Sun's surface field is not maintained in the solar wind.
- Observation of large-amplitude directional fluctuations in the heliospheric magnetic field over the poles.
- Observation of a smaller-than-predicted influx of cosmic rays at high latitudes.
- First observations of the three-dimensional structure of Corotating Interaction Regions (CIRs).
- Discovery of a new class of coronal mass ejections (CMEs) observed at high latitudes.
- First-ever remote sensing of kilometric Type III radio bursts from a high-latitude vantage point.
- Observation of recurrent (~26-day) increases in the flux of low-energy (MeV) particles up to high latitudes, suggesting non-local acceleration followed by latitudinal transport.
- Observation of recurrent (~26-day) modulation (decreases in flux) of high-energy (>100 MeV/n) particles extending to all latitudes.
- Observation of interstellar dust grains and submicron particles, the latter possibly originating in the zodiacal cloud.
- (Tentative) identification of solar p- and g-modes through time-series analysis of energetic particle flux data.
- First characterisation of the interplanetary medium in the ecliptic between 1 and 5 AU near solar maximum.
- First in-situ measurement of interstellar neutral helium flow parameters.
- First measurement of all major species of interstellar pick-up ions.
- First observations of the acceleration of interstellar pick-up ions at interplanetary shocks.
- Observation of recurrent interplanetary dust streams originating in Jupiter's magnetosphere.
- First clear evidence of locally generated Type III radio emission.
- Detailed observations of large solar particle events related to the intense solar activity of March/April 1991.
Jupiter Fly-by Highlights
- First pass through the unexplored dusk sector of the Jovian magnetosphere.
- First direct measurement of the charge states of magnetospheric ions, providing information on the origin and life history of the Jovian plasma.
- Evidence for passage through a polar cap or cusp-like region, with open magnetic flux tubes, deep within the magnetosphere (15 Rj from Jupiter) on the inbound pass.
- Observation of discrete sources of "narrow band kilometric" radio emission distributed along, and rotating with, the Io plasma torus, implying an inhomogeneous structure for the torus.
- Observation of intense beams of energetic ions and electrons streaming inward and outward along magnetic field lines at high latitudes in the dusk sector.
- Observation of short bursts of energetic electrons and associated radio emission, possibly related to Jovian auroral activity.
- Evidence for an unexpectedly strong influence of the solar wind on the structure of the Jovian magnetosphere seen in the tailward deflection of the Jovian magnetic field in the dusk sector.
More than 400 papers discussing these and many other findings from the Ulysses mission have been published since launch, including seven collections of papers in special issues of scientific journals. An eighth is currently in press in Geophysical Research Letters, and it is planned to publish the results from the North Polar Pass Workshop in a special issue of Astronomy and Astrophysics. Ulysses continues to feature prominently at major scientific meetings, most recently at the Solar Wind 8 meeting in Dana Point, California, the IAGA meeting in Boulder and the International Cosmic Ray Conference in Rome.
Ulysses' Second Solar Orbit
With the spacecraft and its scientific payload in excellent condition, Ulysses is all set to embark on its second orbit of the Sun. The ultimate goal of this next phase of the mission is the study of the Sun's polar regions under conditions of high solar activity, culminating in polar passes in 2000 and 2001. Much before this, however, as Ulysses descends slowly in latitude after the northern polar pass, there will be a unique opportunity to make coordinated observations with ESA's SOHO spacecraft, which carries an extensive complement of experiments dedicated to studying the Sun's corona and the solar wind. The period around aphelion (1997-98) will also be of great interest. During this interval Ulysses will spend many months close to the ecliptic at almost constant radial distance (~ 5 AU) from the Sun, enabling the temporal evolution of many interplanetary phenomena to be studied free of concern about spatial variations. Measurements of interstellar pick-up ions (atoms of interstellar gas that have become singly ionised) will also benefit from the "dwell" at moderately large heliocentric distance, since many species are unable to reach the inner solar system.
Ulysses' out-of-ecliptic orbit has a period of 6.2 years, corresponding to approximately half a solar cycle. As noted above, a fortuitous consequence of this is that the high-latitude passes of the second solar orbit will occur close to solar maximum. The conditions in the polar regions are expected to be dramatically different from those encountered during the prime mission. In particular, the rather simple configuration of the corona found at solar minimum, with large coronal holes over the polar caps, will have been replaced by a much more complex arrangement, probably including high-latitude streamers. Transient events (solar flares, coronal mass ejections, etc.) related to the increase in solar activity will dominate, greatly disturbing the underlying structure of the solar wind and influencing the transport of cosmic rays and energetic solar particles. Ulysses is clearly in a unique position, literally and figuratively, to study the evolution of the three-dimensional heliosphere from the current solar minimum to maximum activity conditions. No other space mission in the foreseeable future will address these goals.
In order to involve as wide a community as possible in what, then, is essentially a new mission, it has been decided to establish a Ulysses Guest Investigator programme. Proposals to participate in this programme, which will be open to the international scientific community, will be subject to a joint ESA-NASA review procedure. An important criterion in this review will be whether or not the proposed work adds to the expertise available within the relevant Ulysses Principal Investigator (PI) team(s). Approved Guest Investigators will normally be attached to one or more of the existing PI teams. Following the review, those proposals recommended for selection must obtain funding through their appropriate national funding agencies. In the case of NASA funded proposals, final selection will be made by NASA Headquarters. It is planned to release the Announcement of Opportunity for the Ulysses Guest Investigator Programme in 1996.
Mission and Spacecraft Status
On 13 March, Ulysses crossed the ecliptic plane on the way towards its second polar pass, this time above the Sun's north pole. Compared with the climb to high southern latitudes, the sweep from the south to north polar regions (the so-called "fast latitude scan") occurs relatively quickly, the spacecraft travelling from 80 deg.S to 80 deg.N in less than a year. During this time the distance from the Sun remains rather constant, typically 1.5 astronomical units (225 million km). Ulysses will reach its maximum latitude in the northern hemisphere (80.2 deg.) at the end of July. The mission continues to go well, with all spacecraft subsystems and scientific instruments functioning nominally.
As predicted in the last report, the nutation induced by solar forcing of the axial boom disappeared in the first days of February. The respite was of short duration, however, because the disturbance became visible again in early April as expected. The build-up is being closely monitored using sophisticated software tools developed by the operations team, and the onboard Conscan system is maintaining the required degree of Earth-pointing. The forcing function for the period of nutation in the northern hemisphere is characterised by a slightly higher amplitude than the previous period, but no special problems are foreseen. It should be noted, however, that successful Conscan operations depend on the presence of a (nearly) continuous uplink signal from the NASA Deep Space Network stations at Goldstone and/or Madrid.
The Ulysses spacecraft passed through superior solar conjunction on 5 March, a few days before its perihelion and transit through the ecliptic plane. The conjunction geometry was very unusual due to Ulysses' high-inclination orbit. The radio ray path from the spacecraft to the Earth swept through all solar latitudes from the south pole to the equator, at distances of between 22 and 33 solar radii from the Sun. This provided a unique opportunity to perform a Radio Science experiment to sound the solar corona, enabling density and other measurements to be made. The second such experiment, this was in fact the last coronal sounding that will be carried out using Ulysses, owing to diminishing onboard power resources. Quick-look data show that the electron density in the corona increased, as expected, from pole to equator. One of the surprises was a strong density enhancement within a narrow band of latitude, probably caused by the ray path's passage through a coronal streamer.
Recent solar wind observations show clearly that Ulysses left the high-speed flow from the southern polar coronal hole (in which it had been continuously immersed for almost 18 months) at the beginning of February. Because of the much higher rate at which solar latitude is scanned during the pole-to-pole segment of the trajectory (20 deg. per solar rotation as compared with 2 deg. per solar rotation prior to the southern polar pass), fewer of the characteristic recurrent features seen in the solar wind at mid- and low latitudes on the way to the south pole are evident at comparable latitudes near perihelion. In this respect, the heliospheric current sheet (HCS) that separates oppositely directed magnetic fields from the two solar hemispheres, and which shows a variable tilt with respect to the Sun's rotation axis, is also important. With the continuing decrease in solar activity levels, the HCS has become nearly equatorial. Compared with the final crossing of the HCS prior to the south polar pass at 30 deg.S, the first equatorward crossing occurred much closer to the ecliptic, at 10 deg.S. Since the HCS also separates solar wind flows from the two hemispheres, the current near-equatorial configuration also influences the solar wind stream structure seen at Ulysses. At the time of preparing this report, Ulysses had not yet climbed poleward of the HCS in the northern hemisphere, indicating a possible asymmetry in the maximum extent of the HCS in the two hemispheres.
An intriguing new result to emerge from a detailed spectral analysis of the time series of charged particle fluxes measured on Ulysses is the possible correspondence of spectral components found in the frequency range 1 to 140 microhertz to predicted solar g-modes. At higher frequencies, spectral lines found in the particle data appear to correspond to optically-detected p-modes. The physical interpretation of these findings is not straightforward. although it has been suggested that the solar oscillations may propagate into interplanetary space via hydromagnetic waves which in turn modulate the particles.
These and other new results from the mission were presented in a Special Session at the European Geophysical Society's April meeting in Hamburg. The collection of papers summarising the findings from the south polar pass to be published in Science is scheduled to appear in May, before the Spring meeting of the American Geophysical Union. Future publications highlighting Ulysses will include special issues of Annales Geophysicae and Astronomy and Astrophysics. In addition to the regular Science Working Team meetings, it is planned to hold a North Polar Pass Workshop in late October, to be organised by the JPL project for the Ulysses investigators. This workshop will focus on selected scientific topics related to Ulysses high-latitude results, with less emphasis on near-real time data than at last year's First Polar Pass Workshop in Estec.
The Ulysses mission was one of several on-going heliospheric science missions involving NASA to undergo a Reconfirmation Review in Washington at the end of March. The reviews were aimed at seeking ways of reducing the overall cost to NASA of operating these missions in future and of funding the selected science teams, while at the same time providing wider opportunities for scientific participation. In the case of the Ulysses review, in which the ESA Project Scientist participated, the Board was unanimous in judging the scientific performance of the mission to date and the goals for the second solar orbit to be of the highest calibre. In formulating its recommendations, the Board clearly recognised the constraints implied by the international nature of the mission, in particular the need to adhere to the terms of reference of the existing MOU.
Mission and Spacecraft Status
On 13 September, Ulysses passed a major milestone on its journey of exploration over the poles of the Sun when the spacecraft reached its maximum latitude in the southern hemisphere, 80.2 degrees south of the Sun's equator at a distance of 2.3 astronomical units (345 million km) from the Sun. Ulysses is now slowly climbing back towards the ecliptic, en route to the north polar regions which it will explore in mid-1995. The mission continues to run smoothly, and all spacecraft subsystems and all instruments continue to work well.
Mission operations have been nominal during the reporting period. Starting on 11 August, the spacecraft is tracked almost continuously from either NASA's DSN Canberra complex in Australia or the ESA Kourou station in French Guiana. These measures were taken in anticipation of the predicted onset of the nutation-like motion which disturbed the spacecraft early in the mission. In this mode of operation, active damping is achieved through use of the onboard Conscan system, which maintains the high-gain antenna Earth-pointing within the preset limit (0.25 degrees). This in turn requires a continuous uplink signal from the ground. Geographical coverage limitations at Canberra (the only DSN complex in the southern hemisphere) made it necessary to include an extra station to ensure almost continuous coverage. At the time of reporting, no evidence for the return of the disturbance has been found.
The mission's scientific activities also reached a peak in the week of 13 September, when a 3-day Polar Pass Workshop attended by more than 80 Ulysses investigators was held in Estec. For the duration of the Workshop, the Estec Conference Centre was transformed into an active scientific "laboratory", whereby each experiment team brought a variety of computer equipment to enable team members to work on their latest data on site at Estec. The Workshop programme comprised both plenary and working sessions, and provided ample opportunity for discussion of the key scientific questions arising from the high-latitude observations of Ulysses. The highly successful Workshop was followed on 16 September by an equally successful Press Day in Estec. Media interest in the Ulysses south polar pass was high, resulting in excellent coverage of the event in the European press.
The polar passes of Ulysses take place near the minimum in the current activity cycle of the Sun. The structure of the corona near solar minimum is dominated by the appearance of large coronal holes at the north and south poles with relatively few transient disturbances. From remote sensing observations over many years (utilising, for example, the scintillation of distant radio sources), it was expected that Ulysses would encounter fast solar wind from the coronal holes over the poles. Fast streams of solar wind are also observed in the ecliptic at times when coronal holes extend to low latitudes. Observations from Ulysses, the first ever to be made in situ in the solar wind flowing from the polar caps, have confirmed this expectation. The solar wind speed at Ulysses has remained high (typically 750 km/s) and relatively constant throughout the south polar pass.
This continuous exposure to fast solar wind has enabled Ulysses to study the characteristics of the flow in unprecedented detail, leading to a very clear understanding of the fundamental differences between fast and slow wind. Fast wind from the poles has a lower-temperature source than the slower wind at the equator, and has a different chemical composition.
Ulysses measurements at middle latitudes, where both slow and fast wind were sampled once per solar rotation, have also shown that the boundaries between these two kinds of solar wind are quite sharp and well-defined even at the relatively large distance of Ulysses. Even more surprising is the degree to which the "temperature boundaries" and the "composition boundaries" observed by Ulysses match, since the former must be established in the corona, whereas the latter are created in the chromosphere, below the solar atmosphere. This apparent relationship between conditions in the corona and processes in the chromosphere is expected to eventually shed light on the still-unanswered question as to how the solar wind is created.
A surprising result to emerge from the south polar pass concerns the nature of the heliospheric magnetic field over the poles. It was expected that Ulysses would find evidence of a dipole-like field, similar to a bar magnet, with a clear south magnetic pole. This expectation was based on an extrapolation of the Sun's surface (photospheric) magnetic field, as measured from the Earth using the Zeeman splitting of certain spectral lines. The surface field at solar minimum resembles a dipole with its axis tilted by 10-20 degrees with respect to the Sun's rotation axis. What Ulysses found, however, was a rather uniform heliospheric field with no concentration of magnetic flux - the signature of a dipole - at high latitudes. This result, still not fully understood, will inevitably lead to improvements in our models of the heliospheric magnetic field.
As mentioned in previous reports, cosmic ray measurements during the south polar pass were awaited with great interest. In keeping with the initial indications at moderately high latitudes, the predicted influx of cosmic ray particles over the pole was not observed. In fact, Ulysses detected an increase of at most a factor of two in the flux of cosmic rays over the south pole with respect to the fluxes measured in the ecliptic, compared with a predicted increase of 10 times or more. The absence of a "cosmic ray funnel" is puzzling, although the answer may be related to another Ulysses discovery. The magnetometers on board the spacecraft have shown that the magnetic field over the south pole is less smooth than was generally expected. A wide variety of fluctuations are present, including trains of waves with periods up to 20 hours. It is quite possible that these waves are able to scatter the incoming cosmic ray particles, making the "funnel" less effective.
A collection of papers summarising the initial results from the south polar pass will be published as a special section of the 19 May issue of SCIENCE magazine.
In line with the decision by the SPC at its June 1993 meeting to approve the continuation of the Ulysses mission beyond 30 September 1995 (ref. ESA/SPC/MIN/67), and the assurance given by the NASA Associate Administrator for Space Science to ESA's Director of the Scientific Programme at the recent IACG meeting in Austria that NASA will continue its participation, planning of the scientific operations betond the northern polar pass has begun. These activities include participation in the definition of IACG campaigns involving, for example, SOHO.