Ulysses Jupiter Flyby - Scientific Results
R.G. Marsden and K.-P. Wenzel
ESA Space Science Department, ESTEC, Noordwijk, The Netherlands
(This article appeared in ESA Bulletin No.72 November 1992)
The primary goal of the Ulysses mission, a joint venture of ESA and NASA launched in October 1990, is to explore for the first time the region of space above the poles of the Sun. Although the importance of this exploration to our understanding of the Sun's environment, the heliosphere, has long been recognised, the practical implementation of such a mission has, until relatively recently, been impossible. While we are able to place satellites into a polar orbit around the Earth without much difficulty, the energy needed to launch a space probe into a polar orbit around the Sun is far greater. So much energy is required in fact, that even with the powerful launch vehicles available today, direct injection from the Earth cannot be achieved. This is because the Earth itself orbits the Sun at a speed of 30 km/s in a plane perpendicular to the desired solar polar orbit. The energy imparted to a space probe must cancel out this motion in addition to providing the correct polar trajectory.
A polar orbit can be achieved, however, by taking advantage of a gravity assist by another planet. Jupiter is the nearest body capable of meeting the requirements. The need to make use of Jupiter in order to carry out its primary mission has resulted in the recent flight of the ESA-built Ulysses spacecraft through the Jovian magnetosphere. Even with this gravity assist, the combined power of the space shuttle and three upper-stage rockets were needed to send the 370 kg space probe on its way. As it left the confines of the Earth's gravitational field, Ulysses was travelling at 11.3 km/s, making it the fastest interplanetary spacecraft ever launched.
Ulysses arrived at Jupiter 16 months after departing from Earth, having travelled nearly 1 billion kilometers in the ecliptic. (See ESA Bulletin No. 67, August 1991, for a report on the first scientific results from the in-ecliptic phase of the mission). Closest approach to the planet occurred at 12:02 UT on 8 February, 1992. As explained above, the primary aim of the flyby was to place the spacecraft in its final heliocentric out-of-ecliptic orbit with a minimum of risk to the onboard systems and scientific payload. Scientific investigations at Jupiter are a secondary objective of the mission. Nevertheless, the opportunity to study Jupiter's magnetosphere was exploited to the greatest extent possible. The results exceeded all expectations of the scientists involved.
Jupiter is a strongly magnetised, rapidly rotating planet. Its magnetosphere is the largest object in the solar system, a fact reflected in the long interval of 12 days from 2 to 14 February (days 033 to 045 of 1992) that it took for Ulysses to travel through it. The large Galilean satellites are embedded within the magnetosphere and Io is known to be a prolific source of ions and neutral particles (Fig. 1). Ions, predominantly of sulfur and oxygen, are distributed around the orbit of Io to form a large torus. Electrons and ions from Io, Jupiter's ionosphere and the solar wind are all present and are transported throughout the magnetosphere. A substantial fraction of these particles are accelerated to extremely high energies to form intense radiation belts. Upstream of the magnetosphere, in the free-streaming solar wind, a detached bow shock forms which slows the solar wind and allows it to be deflected around the magnetosphere. A wide variety of complex physical phenomena are available for study.
The inbound trajectory (Fig. 2) was rather similar to those of the four spacecraft which flew past Jupiter previously: Pioneer 10, 11 (1972, 1973) and Voyager 1, 2 (1979). In contrast to these missions, however, Ulysses reached high latitudes (40 deg. north of Jupiter's equator) near closest approach. A unique aspect of the Ulysses flight path was the outbound passage through the hitherto unexplored dusk sector (18:00 hours local time) of the magnetosphere, this time at high southern latitudes. Another unique aspect of the flyby was the penetration of the Io Plasma Torus (IPT), a few hours after closest approach, in a basically north-south direction which contrasted with the nearly equatorial Voyager 1 traversal. In addition to this direct penetration, the spacecraft radio signal passed through the IPT for a significant length of time making it possible to probe the electron density distribution in the Torus.
Although the instruments that make up the scientific payload are optimised for the conditions encountered in the solar wind, including their orientation on the spinning spacecraft, they have produced a wealth of new information relating to the Jovian magnetosphere. In this report we summarise some of the initial findings. As is the case for the primary mission, many of the observations made in the magnetosphere by the different experiments are complementary in nature, making a correlative approach the most fruitful in terms of data interpretation.
Plasma and Magnetic Field
A fundamental contribution provided by the plasma (SWOOPS) and magnetic field (FGM/VHM) experiments is the identification of the boundaries between the various magnetospheric regions encountered during the flyby (Fig. 3). This "road map" is a useful tool that helps other experiments to place their observations in the correct context. On 2 February, almost a week before closest approach, Ulysses crossed the Jovian bow shock at a distance of 113 Jupiter radii (1 Rj = 71,398 km) from the planet (Table 1). The inbound crossing occurred somewhat earlier than expected based on previous observations by the Voyager spacecraft. A possible interpretation is that the solar wind ram pressure was low, allowing the magnetosphere to temporarily "inflate", causing the bow shock to "stand off" further out from the planet. The magnetopause, the outer boundary of the magnetosphere, was first encountered only four hours after the bow shock crossing, at a distance of 110 Rj. The apparent proximity of these two boundaries, which are typically separated by 20 Rj, also suggests that they were moving rapidly outwards at that time. On the outbound leg, multiple magnetopause and bow shock crossings were observed as these boundaries moved inwards then outwards across the spacecraft. This is again indicative of the "elasticity" of the magnetosphere in response to changing solar wind conditions.
Other results to emerge from magnetic field observations (Fig. 4) include the previously unknown configuration of the dusk side field, which is strongly swept back towards the magnetotail, and the realisation that large-scale current systems are very important in determining the configuration and dynamics of the magnetic field.
Radio and Plasma Waves
Jupiter is a prolific source of natural radio waves, emitting at many wavelengths. The unique direction-finding capability and high sensitivity of the Ulysses radio and plasma wave (URAP) experiment have provided new insights and clues as to the origin of these radio signals (Fig. 5). For example, the so-called "narrow-band kilometric" (nKOM) radiation has been found to originate from discrete, long-lived sources that are located in the outer regions of the Io Plasma Torus, and which rotate around Jupiter at slightly different rates. Ulysses observations of the hectometric radiation (HOM) revealed narrow latitudinal beaming along the magnetic equator, and provided additional constraints on existing models for the source of this radio emission. Several bursts of radio emission showing a characteristic rapid drift in frequency, so-called "Jovian type III" events were detected with Voyager. With Ulysses, many events of this type have been recorded and they appear to be a major component of Jupiter's radio spectrum.
At the lowest frequencies, the Jovian continuum emission has been observed by URAP at large distances from the planet. Both the frequency range and intensity of the continuum have been seen to vary with solar wind ram pressure, thus providing a unique long-term remote monitor of solar wind conditions at Jupiter.
The Ulysses solar wind ion composition (SWICS) experiment has provided important new data on the composition of the magnetospheric plasma, in particular information on the charge states of the various ions (Fig. 6). These measurements give new insights into the sources and "life history" of the Jovian plasma. Material from three sources, the solar wind, the planet itself and the volcanic satellite Io (Iogenic ions) can be traced by observing ions specific to each source. Large-scale mixing was observed to occur, with significant numbers of solar wind and Iogenic ions being found in all regions investigated. The implication is that solar wind plasma penetrates deep into the magnetosphere and Iogenic ions travel outwards to its outer reaches, both at low and high latitudes. These results provide important tests for models of plasma circulation, transport and loss from the magnetosphere.
Io Plasma Torus
The Io Plasma Torus (IPT), a doughnut-shaped ring of plasma rotating with the planet at the orbit of Jupiter's moon Io, was an object of special interest during the flyby. Its properties were measured both directly by the URAP instruments, and also remotely by the Ulysses Radio Science team. The IPT consists largely of ionised oxygen and sulfur atoms released as a neutral cloud by Io's volcanoes. The Ulysses measurements, taken a few hours after closest approach as the spacecraft crossed Jupiter's magnetic equator, indicate that the electron density of the IPT in the current epoch matched model predictions based on the older Voyager results quite well (Fig. 7). On the other hand, the longitudinal distribution of plasma seen by Ulysses showed asymmetries not expected from the Voyager data, which, as mentioned in the introduction, were taken in a different region of the IPT.
The energetic particle intensities measured by the Ulysses instruments during the flyby (Fig. 8) were generally lower than observed by the Voyager spacecraft. A major discovery during the outbound pass was the existence at high latitudes of very strong counter-flowing streams of electrons and ions, constituting large currents that apparently feed into the auroral regions. Measured principally by the HI-SCALE experiment, these field-aligned particle beams are tightly confined to magnetic field lines which, as noted above, appear to be swept strongly tailward. In the same regions, the COSPIN experiment observed periodic bursts of MeV electrons flowing away from the planet (Fig. 9). Preliminary estimates indicate that these bursts may represent a significant fraction of the population of relativistic electrons found in interplanetary space. Data from many of the Ulysses experiments point to the fact that the dusk-side magnetosphere, where the fields and plasmas rotate from the compressed day side into the magnetotail, is highly dynamic. Furthermore, signatures in the energetic particle data indicate that the high latitude region of the magnetosphere appears to be dominated by the interaction of the solar wind with the planet's magnetic field.
Polar Cap Region
Because of its high-latitude trajectory, Ulysses was able to investigate the poleward extent of the Jovian radiation belts that contain durably trapped energetic electrons and ions. A surprising finding was that even close in to the planet (approximately 9 Jupiter radii), Ulysses apparently made an excursion out of the radiation belts at magnetic latitudes of only 48 degrees. Signatures in the data from many of the instruments are consistent with Ulysses having passed through a region in which magnetic field lines from the Jovian polar cap at one end were connected to interplanetary magnetic field lines at the other end. For example, the energetic particle sensors registered counting rates close to interplanetary background levels, while the SWOOPS plasma instruments noted a simultaneous drop-out in the magnetospheric electron population for a period of 1.5 hours. URAP DC electric field measurements indicate a drop in the plasma flow speed to very low levels.
An important accompaniment to the flyby was a set of supporting observations carried out by ground-based observers, particularly those involved in the Jupiter Watch programme, as well as observers making use of Earth-orbiting spacecraft including the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE). Using ESA's Faint Object Camera onboard HST, European scientists obtained an image of the polar aurora surrounding Jupiter's north pole just 15 hours after Ulysses' closest approach (Fig. 10). This type of remote sensing of emissions from Jupiter's auroral regions, and the IPT, has helped to establish the context in which the flyby took place.
Although the study of the Jovian magnetosphere was not a primary mission objective, the recent flyby of the giant planet by Ulysses has produced a wealth of new observations that constitute a major contribution to our understanding of this complex and dynamic plasma environment. Following the flyby, all spacecraft systems and the scientific instruments were checked out thoroughly and found to be in good health. Having therefore emerged from Jupiter's hostile radiation environment unscathed and perfectly on course, Ulysses has now begun the most exciting phase of its mission. Moving out of the ecliptic towards the Sun's southern polar regions, it has started its exploration of the high-latitude heliosphere. Helped on its way by the immense gravitational field of Jupiter, Ulysses will pass over the southern pole in September 1994 and continue on to the northern polar regions which it will reach one year later. Based on the exciting results obtained to date both during the in-ecliptic phase and at Jupiter, we can expect a rich scientific harvest from the primary phase of the mission, as well as some surprises.
Table 1. Sequence of Events during the Ulysses Jupiter Flyby. Event Time Distance (Day/Hour/Min) (Rj) ----------------------------------------------------------------- Bow Shock Crossing (In) 033/17:33 113 Magnetopause Crossings (In) 033/21:30-035/04:00 110-87 Magnetodisc/Plasmadisc 036/06:30-037/22:00 67-36 Crossings High Latitude Polar Cap 038/22:30 15 (or possibly Cusp) 039/06:30 8.7 Closest Approach 039/12:02 6.31 Observations of Io Plasma 039/13:00-18:00 6.4-9.0 Torus Observation of Field Aligned 041/01:00-043/13:00 35-82 Currents, Electron and Ion Streaming Magnetopause Crossings (Out) 043/13:57-045/21:40 83-124 Bow Shock Crossings (Out) 045/00:37-047/07:52 109-149 -----------------------------------------------------------------
Figure 1. Schematic of Jupiter's magnetosphere. The solar wind, approaching from the left, is deflected around the magnetosphere by the bow shock. The outer boundary of the magnetosphere, called the magnetopause, is indicated. Major structural features inside the magnetosphere are shown.
Figure 2. The Ulysses trajectory past Jupiter. The open circle represents the point of closest approach at 6.3 Jovian radii (450,000 km) from the centre of the planet. Vertical lines denote intervals of 3 hours relative to closest approach.
Figure 3. Colour-coded spectrogram of the entire 15-day Ulysses flyby from the SWOOPS electron plasma instrument. Electron energy spectra summed over all look directions are displayed using the colour bar shown on the right to code the count rate.
Figure 4. The magnetic field magnitude measured by the FGM/VHM experiment during (a) the inbound pass and (b) the outbound pass is plotted versus radial distance from the planet. The dashed line superimposed on the data denotes the predicted model field.
Figure 5. Overview of URAP radio and plasma wave data during the flyby displayed as frequency vs. time dynamic spectra, with relative intensity indicated by the colour bar on the right. (A): 16 day overview centred on Closest Approach (CA, day 039). (B): two typical Jovian rotations before closest approach beginning at 05:00 SCET on 5 Feb. (C): 24 hour period centred on CA including passage through the Io Plasma Torus. (D): 24 hour period on the day of the inbound bow shock and magnetopause crossing. (E): 24 hour period containing some of the outbound magnetopause crossings.
Figure 6. SWICS ion composition data plotted in the form of a mass vs. mass-per-charge matrix of solar wind (top) and Jupiter magnetospheric plasma (bottom). The change in charge state and elemental composition is striking.
Figure 7. Electron densities within the Io Plasma Torus as determined by the URAP experiment. For comparison, model predictions based on Voyager data are also shown.
Figure 8. Overview of HI-SCALE measurements of ions and electrons in Jupiter's magnetosphere displayed as an energy spectrogram (upper panel) and counting rates (lower panel).
Figure 9. Data from the COSPIN experiment showing a quasi-periodic sequence of electron bursts observed on the outbound pass in the dusk sector of the magnetosphere. Shown for comparison are the simultaneous proton measurements.
Figure 10. False colour image of the northern polar region of Jupiter observed with ESA's Faint Object Camera onboard the Hubble Space Telescope a few hours after the Ulysses flyby.