SWOOPS/Electron - User Notes

SWOOPS Contact:

Dr. Bruce E. Goldstein
Address: MS-169-506
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109

Phone: (1) 818 354-7366
Telefax: (1) 818 354-8895

E-mail: bgoldstein@jplsp2.jpl.nasa.gov

                        THE ELECTRON EXPERIMENT

                             TABLE OF CONTENTS



The SWOOPS (Solar Wind Observations Over the Poles of the Sun) experiment has
two electrostatic analyzers, one for positive ions and one for electrons. The
instrument is fully described in: The Ulysses Solar Wind Plasma Experiment, S.
J. Bame, D. J. McComas, B. L. Barraclough, J. L. Phillips, K. J. Sofaly, J. C.
Chavez, B. E. Goldstein, and R. K. Sakurai, Astronomy and Astrophysics
Supplement Series, Ulysses Instruments Special Issue, Vol. 92, No. 2, p.
237-265, 1992. The electron and ion analyzers are separate instruments that
operate asynchronously. For this reason, the data from the two sensors are
submitted separately to the NSSDC. This document describes the electron
analyzer and the data submitted to the NSSDC for that analyzer; the ion
experiment is described in a comparable document that accompanies the ion data.


The SWOOPS electron spectrometer is a 120-degree spherical section
electrostatic analyzer which measures the 3-d velocity space distributions of
solar wind electrons. In its normal solar wind mode, the instrumental energy
range is 1.6 to 862 eV in the spacecraft frame. Since the spacecraft generally
charges to +2 to +15 volts, 2 to 15 eV is subtracted from the measured energies
(electrons measured at energies below the spacecraft potential are electro-
statically trapped photoelectrons). The beginning of scientifically useful
SWOOPS data is at the beginning of Day 322 of 1990.

The electron instrument is subject to a "sleep mode" triggered by changes in
the spacecraft configuration. The first time this mode occurred it caused a
long gap in the electron (but not the ion) data: from 0621 UT on day 346 of
1990 until 2052 UT on day 362. Subsequently, procedures were developed to
recognize, correct, and prevent this sleep mode, thus minimizing its impact.
However, there are occasional gaps in the electron data when the SWOOPS ion
sensor, and other Ulysses experiments, were returning data.


Each spectrum takes 2 minutes to accumulate, but telemetry takes longer. When
the spacecraft is being actively tracked and is returning data at its highest
bit rate, spectra are returned every 2.3 minutes (low angular resolution mode)
or every 5.7 minutes (high resolution mode). During playback of stored data,
the spectral repetition rate is every 4.7 minutes (low angular resolution) or
every 11.3 minutes (high resolution). These modes will be described in the next
paragraph. For most of the mission, the instrument has been in high angular
resolution mode, resulting in spectra every 5.7 minutes when the spacecraft is
being tracked, or every 11.3 minutes for playback. The amount of spacecraft
tracking depends on the mission phase.


The electron analyzer is provided with a 22-level high voltage supply to cover
a range of ion energies from 0.8 to 862 eV. At any given time, either the top
20 or bottom 20 of these voltage levels are used. Except from a period from
instrument turn-on in November 1990 through December 3, 1990, the instrument
has been in high-energy mode throughout the mission, resulting in an energy
range of 1.6 to 862 eV. Prior to that time, the instrument alternated between
low-energy (0.8 to 454 eV) and high-energy (1.6 to 862 eV) spectra.

The analyzer uses 7 channel electron multipliers (CEMs) to count electrons
discretely over 95% of the unit sphere in look direction. For telemetry
conservation, 2 out of every 3 spectra are "two-dimensionalized" onboard the
spacecraft, that is the count rates are averaged over all 7 CEMs. These 2-d
spectra thus return electron counts as a function of energy and spacecraft spin
angle. The full 3-d spectra return counts as a function of energy, spacecraft
spin angle, and polar angle (measured from the spacecraft spin axis, which
points at Earth).

There are two angular resolution schemes which are ground commanded. In the
high resolution scheme, which has been used for most of the mission, the 3-d
spectra incorporate 32 spin-angle steps, for a total spectral content of 20
energies x 32 azimuths x 7 polar angles. The 2-d spectra incorporate 64 spin
angle steps, for a total of 20 energies x 64 azimuths x 1 CEM-averaged polar
angle (90 degrees from the spin axis). In the low resolution scheme, both 2-d
and 3-d spectra incorporate only 16 spin angle steps, but provide a higher
spectral repetition rate, as described in the previous section.

Because spacecraft potential effects vary with polar angle, and the 2-d spectra
sum over polar angle, it is not possible to correct fully for these effects
using the 2-d spectra. The SWOOPS team has not found the 2-d spectra to be
useful and they are not included in the NSSDC submission. During the highest
spacecraft bit rate periods and highest instrument angular resolution, one 3-d
spectrum is returned every 17.1 minutes.


Calculation of the electron moments are strongly effected by the presence
and variability of the assymetric spacecraft potential sheath.  Ulysses
electron moments have been calculated to minimize these effects, however,
some uncertainties remain in the inversion of these observations.  Our first
step in the electron data reduction process is determination of the bulk,
scalar spacecraft potential.  This is done by identifying inflections in
the angle-averaged energy spectra.  The spacecraft potential averages +6V,
with higher values for low-density plasma and lower (but still positive)
values for high-density plasma. A second inflection is also identified in
the spectra, corresponding to the break between thermal ("core") and
suprathermal ("halo") populations. The count rate arrays are then corrected
for spacecraft potential and converted to phase-space density arrays using
the "plane-parallel correction" (e.g., Scime et al., JGR, p. 14769, 1994),
a correction which attempts to unfold not only the energies but also the
direction of motion of the electrons from thier observation point at the
instrument apperture to well outside the spacecraft sheath.

At high heliographic latitudes scattering of light into the sensor was
intermittently observed, adversely effecting calculations of halo electron
properties.  When this condition occurs, it is patched by interpolation
from neighboring data measurements that are not contaminated by light.

Plasma moments are then calculated by numerical integration of the
velocity- weighted ion distributions. A total integration is performed from
the spacecraft potential (corresponding to zero energy solar wind
electrons) to the instrumental energy limit. Analogous core and halo
integrations are performed for the parts of the distribution above and
below the core-halo energy break point. Since the first few eV above the
spacecraft potential are contaminated with photoelectrons on non-radial
trajectories, it is necessary to use a biMaxwellian fit to the core
distribution to fill in this part of the distribution; the total and core
integration results are corrected based on this fit.

The integrations produce density, temperature components, velocity, and
heat flux. Uncertainties in the spacecraft potential and sheath
configuration create errors in the density and temperature calculations.
These problems are particularly severe when the solar wind is rarefied and
cold, making it difficult to separate the photoelectron and thermal
distributions. The halo density and core and halo temperatures are much
less affected by the photoelectron effects than is the core density. The
SWOOPS ion instrument provides much more accurate measurements of solar
wind bulk density.  Therefore, assuming charge nutrality, we use
interpolated ion densities (Np + 2Na) minus measured halo densities to fill
the core electon density column in these data.

Finally, the URAP experiment aboard Ulysses operates a radio-frequency
sounder which can also distort the low-energy electron distributions.
Spectra measured during sounder operations are not included in the NSSDC
data submission.


The data provided to the NSSDC are the total, core, and halo electron densities
and scalar temperatures at full instrumental time resolution, plus spacecraft
position. The data submitted to the NSSDC for the SWOOPS electron experiment
was replaced in its entirety in mid-May, 1999. The earlier data set did not
have the several spacecraft potential and scattered light corrections and
software filters and is not as reliable as the current submission.  We
strongly reccommend the use of these revised calculations in all future

The time specified in the file is the center of each 2-minute spectrum. This
time roughly corresponds to the center of the core distribution; the most
appropriate time for the halo properties is roughly 30 seconds later.

One file per month is provided, with a naming scheme as follows:
U97244BAMELE.DAT corresponds to data starting on day 244 of year 1997. In the
21st century, years will continue to be provided in two digit format; e.g.,
U01244BAMELE.DAT will correspond to data starting on day 244 of year 2001.

The files can be opened and read as follows:

      open (3, file='U97244BAMELE.DAT', status='old')

c      iyr           - year
c      idoy          - day of year (Jan 1 = 001)
c      ihr           - hour, UT
c      imin          - minute, UT
c      isec          - second, UT
c      sunsc         - sun-spacecraft distance, AU
c      hlat          - heliospheric latitude of spacecraft, degrees
c      hlong         - heliospheric (Carrington) longitude of spacecraft, deg
c      den          -  total number density of electrons per cubic cm
c      denc          - core electron number density per cubic cm
c      denh          - halo electron number density per cubic cm
c      tempe         - total electron temperature, Kelvins
c      tempc		      - core electron temperature, Kelvins
c      temph         - halo electron temperature, Kelvins

      read (3, 1) iyr,idoy,ihr,imin,isec,sunsc,hlat,hlong,
     >            den,denc,denh,tempe,tempc,temph

1     format(1x,i2,1x,i3,3(1x,i2),f7.4,2f7.2,6e13.5)

Spectra with known bad density or temperatures, 2-d spectra, or spectra taken
when the URAP sounder is operating are not submitted to the archives.


For information on acquiring other types of data not provided to the the NSSDC,
contact the Principal Investigator, Dr. David J. McComas, at Southwest Research
Institute, dmccomas@swri.edu, 210-522-5983. For information on the reduction
and analysis of data from the positive ion and electron experiments, contact
Dr. Bruce E. Goldstein at the Jet Propulsion Laboratory,
bgoldstein@jplsp2.jpl.nasa.gov, 818-354-7366.