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Astronomy and Astrophysics Supplement Series, Ulysses Instruments Special
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Astron. Astrophys. Suppl. Ser: 92, 237-265 (1992)

The Ulysses solar wind plasma experiment

S.J. Bame^1, D.J. McComas^1, B.L. Barraclough^1, J.L. Phillips^1, K.J.
Sofaly^1, J.C. Chavez^2, B.E. Goldstein^3, and R.K. Sakurai^3

^1 University of California, Los Alamos National Laboratory, Los
Alamos, NM 87545, USA
^2 Sandia National Laboratories, Albuquerque, NM 87185, USA
^3 California Institute of Technology, Jet Propulsion Laboratory,
Pasadena, CA 91109, USA

Received April 8; accepted July 16, 1991

Abstract. -- The Solar Wind Plasma Experiment on Ulysses is accurately
characterizing the bulk flow and internal state conditions of the
interplanetary plasma in three dimensions on the way out to Jupiter.
These observations will continue over the full range of heliocentric
distances and heliographic latitudes reached by the probe after its
encounter with Jupiter and consequent deflection out of the ecliptic
plane. Solar wind electrons and ions are measured simultaneously with
independent curved-plate electrostatic analyzers equipped with
multiple Channel Electron Multipliers (CEMs). The CEMs are arranged to
detect particles at chosen polar angles from the spacecraft spin axis;
resolution in spacecraft azimuth is obtained by timing measurements
with the spacecraft Sun clock as the spacecraft spins. Electrons with
central energies extending from 0.86 eV to 814 eV are detected at
seven polar angles and various combinations of azimuth angle to cover
the unit sphere comprehensively, so as to enable computation of the
pertinent electron velocity distribution parameters. As the average
electron flux level changes with heliocentric distance, command
control of the CEM counting intervals is used to extend the dynamic
range. Ions are detected between 255 eV/q and 34.4 keV/q using
appropriate subsets of 16 CEMs at spin angles designed to provide
matrices of counts as a function of energy per charge, azimuth angle,
and polar angle centered on the average direction of solar-wind flow.
Data matrices are obtained every 4 min when the spacecraft is actively
transmitting and every 8 min during data store periods. These matrices
contain sufficient energy and angle resolution to permit a detailed
characterization of the ion velocity distributions, from which ion
bulk parameters are derived. As the average ion flux intensity changes
with heliocentric distance, the entrance aperture size is periodically
optimized by command selection from a set of seven apertures on a disk
driven by a stepping motor. Changes in the average solar wind flow
direction relative to the Earth-pointing spacecraft spin axis are
accommodated by command selection of the proper measurement matrix
from a set of 11 matrices. In a separate mode of operation and under
favorable conditions, heavy ions of oxygen, silicon, and iron at
various charge levels are resolved.

Key words: Sun: solar poles--solar wind: latitudinal variation--
interplanetary/ heliospheric medium.

1. Scientific objectives.

Ulysses is providing an exciting opportunity to explore and
characterize the heliospheric medium from the ecliptic plane to above
the solar poles at distances from the Sun extending from 1 AU to 5 AU.
Observations made on the way out to Jupiter are extending our
knowledge of the interplanetary medium in the ecliptic plane, gained
from the Pioneer and Voyager flights, while observations made after
Ulysses encounters Jupiter and journeys over the solar poles will be
unique. The primary objective of the Ulysses solar wind plasma
investigation, SWOOPS (Solar Wind Observations Over the Poles of the
Sun), is to investigate and establish bulk flow parameters and
internal state conditions of the solar wind as a function of solar
latitude. Further important goals include studies of radial gradients
of solar wind properties between Earth and Jupiter and investigations
of the solar wind interaction with the Jovian magnetosphere. Among the
important objectives of the SWOOPS investigation are the following:

   - Determine systematic variations in the solar wind bulk flow with
     solar latitude,

   - Examine physical processes important for driving the coronal
     expansion,

   - Investigate variations in the evolution of high speed streams
     with latitude,

   - Investigate changes in the nature of transient disturbances with
     latitude,

   - Determine latitudinal variations in the relative abundance and
     charge state composition of solar wind minor ions,

   - Search for and identify plasma mechanisms regulating the electron
     heat flux, ion beam relative flow velocities, and the He^++ to
     H^+ ion temperature ratio,

   - Survey latitudinal variations in Alfven wave amplitudes and the
     relative numbers of tangential and rotational discontinuities,

   - Determine coronal hole temperatures at which high speed stream
     solar wind Fe ionization states are established,

   - Estimate the electrostatic potential and electron collision
     length dependence on latitude,

   - Identify local heating mechanisms of solar wind ions,

   - Study the interaction of interstellar neutrals with the solar
     wind,

   - Study large amplitude hydromagnetic waves, and

   - Evaluate radial and meridional gradients in solar wind electron
     temperature and anisotropy.

1.1. GLOBAL PROPERTIES OF THE SOLAR WIND.

Much is known about solar wind structure and dynamics in the near-
ecliptic region of interplanetary space from plasma measurements made
during the past 30 years. Very little is known about physical
conditions in the polar regions of the solar corona and solar wind.
For example, information concerning the variation with heliographic
latitude of so fundamental a flow parameter as the solar wind mass
flux is lacking. Similarly, little is known about latitude variations
of the energy and momentum fluxes, the internal plasma state, and
hydromagnetic wave field. Yet these parameters are important in
determining the physical state of the polar corona as well as the size
and shape of the heliospheric cavity which separates the Sun from the
local interstellar medium. They are also important for understanding:
(a) the physical processes dominant in accelerating and modulating
energetic charged particles, (b) the interaction between the Sun and
the neutral component of the local interstellar medium, (c) the nature
of the interplanetary scintillation of radio sources, and (d) the
evolution of cometary bodies and dust grains orbiting the Sun in
interplanetary space. A prime objective of the SWOOPS investigation is
to fill the present gap in our knowledge of the local solar
neighborhood by providing three-dimensional (3D) measurements of the
solar wind electrons, and the H, He, and heavier ion components at
latitudes ranging from the solar equator to near the solar poles.

1.2. ENERGETICS AND ACCELERATION OF THE SOLAR WIND.

A fundamental goal of current research is to understand the physical
processes that maintain a hot corona and accelerate coronal plasma
into interplanetary space. It has been necessary to draw inferences
concerning these processes from observations made far from the Sun and
generally in the ecliptic plane, where dynamical processes make it
difficult to separate the basic solar wind states from states altered
during transit through interplanetary space. Ulysses will make
possible studies of the very simplest corona/solar wind regions, such
as the expansion from long-lived polar coronal holes in which stream-
stream interactions should be absent and in which the magnetic field
should have a relatively simple configuration, extending outward
nearly radially. Observations of the density, flow speed, electron and
ion internal states, the hydromagnetic wave field, and the minor ion
composition in the polar solar wind with SWOOPS will provide
information relating to the solar wind acceleration that should be
applicable to all latitudes including the ecliptic plane.

1.3. NONLINEAR MHD DISTURBANCES - LARGE SCALE SOLAR WIND STRUCTURES.

The large scale solar wind structures in the ecliptic are usually high
speed and low -speed streams from coronal holes and the equator-ward
coronal streamer belt. Solar rotation ensures that these flows
interact with each other even when the corona is evolving slowly with
time. The picture is additionally complicated by coronal mass
ejection, CME, events which propel fresh coronal material into the
solar wind.

At the present time little is known about how corotating structures
and transient disturbances interact with each other and evolve at
solar latitudes away from the ecliptic plane. Some evidence suggests
that high-speed streams are bounded on all sides by large shears.
Knowledge of the 3D shapes of shocks and other transient structures in
interplanetary space is needed to gain a global perspective of the
flow of energy from energetic coronal events into the heliosphere. Not
only is the physics of these flow interactions of interest in its own
right, but the global structure of the interactions affects a variety
of related disciplines. For example, energetic particles are modulated
and/or accelerated by their interaction with both transient-driven and
corotating shock disturbances, the heliosphere boundary is buffeted by
these disturbances, and the brightness of comets is thought to be
enhanced by interactions with shock wave disturbances. The long term
observation of these large scale plasma structures away from the
ecliptic plane with the SWOOPS experiment will clarify and enhance the
knowledge that has been gained with in-ecliptic observations.

1.4. LARGE-AMPLITUDE HYDROMAGNETIC WAVES - SMALL SCALE SOLAR WIND
STRUCTURES.

On a time scale of hours, large amplitude solar wind irregularities
are generally collisionless hydromagnetic fluctuations. They may be
either propagating waves or plasma variations that are at rest with
respect to the solar wind. Many such structures can be identified as
Alfven waves, or tangential or rotational discontinuities. The
relation of the amplitudes, polarizations, modes, and spectra of
interplanetary fluctuations to the large scale structure of the solar
wind is of great interest. Acquisition of a global perspective of the
evolution of large amplitude hydromagnetic waves and turbulence in the
solar wind with SWOOPS will contribute not only to a detailed
understanding of the heliosphere, but to an understanding of other
astrophysical objects as well.

The Ulysses survey of the variation of interplanetary microstructure
with heliographic distance in the ecliptic is well underway; the
survey will soon be extended to higher solar latitudes after Ulysses
encounters Jupiter in February 1992. The results of the survey will
provide constraints on theories of the origin, propagation, and
dissipation of interplanetary waves and on theories of their role in
large scale solar wind dynamics.

1.5. INTERNAL STATE OF THE SOLAR WIND PLASMA.

One of the prime objectives of SWOOPS is to establish a global picture
of the internal plasma state of the solar wind. The internal state is
reflected as complex shapes of the ion and electron velocity
distributions. Ion distributions range from isotropic Maxwellian to
multiple streaming distributions. Electron distributions appear to be
bi-Maxwellian with enhanced heat flux extensions along the magnetic
field at higher energies. The distributions can often be modelled in
terms of two relatively convecting components carrying significant
amounts of free energy. This energy can be released by triggering of a
variety of micro-instabilities, leading to wave growth which can react
back on the particle populations. Such kinetic processes regulate the
efficiency with which the plasma conducts heat and transports linear
and angular momentum.

Observations of the internal state of the plasma are relevant to
several physical and astrophysical disciplines. They place constraints
on theories of the coronal expansion. SWOOPS measurement of the
strongly beamed high energy electron component is used as an indicator
of the large scale magnetic topology and may provide a measure of the
interplanetary electrostatic potential at high latitudes. Whether the
beam of high energy protons that is usually present at high speeds
persists at high latitudes is of interest since this beam may cause
local proton heating. Studies of such phenomena can contribute towards
understanding how plasma turbulence develops and its nature in an
astrophysical setting.

1.6. HEAVY ION COMPOSITION OF THE SOLAR WIND.

Under favorable low temperature conditions, determinations of the
solar wind ion composition can be made from E/q measurements alone.
Most of the time, the extensively studied heavy ion, He^++ (also
referred to in this paper as He ions, helium, and helium ions) can be
resolved from the more abundant hydrogen ions, H^+, with great
accuracy. Helium has a widely variable abundance, ranging from <0.1%
to >25%. The lower values are found in low-speed solar wind associated
with the coronal streamer belt and magnetic field reversals, while the
higher values are associated with transient coronal events such as
flare-induced CMEs. Corotating high speed stream flows contain a
nearly constant mid-range He abundance near 4.8%. Routine measurements
to high heliographic latitudes on Ulysses will help to complete the
picture of the global systematics of helium variability.

Although a secondary objective, SWOOPS measurements of heavy ion
species, e.g. Fe^+10, at higher solar latitudes can contribute to
several important problems of solar physics. A problem of particular
interest concerns coronal hole electron temperatures at which the high
speed stream ionization states are established. A determination of the
Fe ionization state in high speed streams provides a measure of this
temperature. Such a determination can be made by fitting the Fe ion
distribution envelope, even when the individual Fe species are not
completely resolved.

1.7. SOLAR-WIND INTERACTION AT JUPITER.

The SWOOPS experiment was not specifically designed to study the solar
wind interaction with Jupiter's magnetic field and the nature of the
Jovian magnetosphere during the Ulysses encounter with that planet,
because first priority had to be given to measuring the high latitude
solar wind ions and electrons that will be explored after the
encounter. However, substantial fractions of the magnetospheric
electrons and ions in the Jovian system will fall within the energy
and angular acceptances of the SWOOPS analyzers, so valuable
observations will be obtained there, particularly in view of the fact
that Ulysses will traverse a different path than those taken by the
predecessor Pioneer and Voyager spacecraft. Benchmark measurements of
the bow shock and magnetopause locations will be made for comparison
with earlier flyby observations and with the measurements that are to
be made later by Galileo. The Ulysses measurements throughout the
Jovian system should contribute to a more complete understanding of
the nature of the interaction at the various interfaces as well as the
nature of the plasma flow within and along the outer flank of the
magnetosphere.

2. The approach adopted for SWOOPS.

2.1. GENERAL.

To satisfy many of the objectives outlined in the previous section, it
was clearly desirable to measure ion and electron 3D velocity
distributions in very fine detail and with high time resolution.
However, ever finer spectral detail is costly in terms of experiment
temporal resolution, weight, power, complexity, and bit rate.
Similarly, ever higher time resolution can be obtained only at the
expense of spectral detail, complexity, and bit rate. The approach to
the SWOOPS design was to attempt to strike a balance between the
conflicting requirements so as to optimize the overall plasma science
yield from the mission and support other Ulysses investigations while
staying within available mission resources. In the following
subsections, the natures of the plasmas to be encountered, which
placed constraints on our design criteria, are examined. These
distributions are, of course, from in-ecliptic measurements, but high
speed solar wind is emphasized since it is expected that at high
latitudes the flows will be somewhat similar to high speed flows
observed in the ecliptic plane.

2.2. HYDROGEN AND HELIUM IONS, H^+ AND He^++.

Within typical low and high speed solar wind flows, the He^++ ions,
which usually have local temperatures four to six times higher than
those of the H^+ ions, can be resolved in E/q spectra into separate
hydrogen and helium components, except in unusual cases like one
discussed below. They are less resolved in interaction regions at the
leading edges of high speed flows, but it is expected that at the high
latitudes to be reached by Ulysses the interactions between high and
low speed streams will be both less intense and less frequent.

A survey of solar wind spectra shows that it is not unusual to find
H^+ velocity distributions that cannot be described adequately by
models consisting of simple extensions to a bi-maxwellian shape which
includes a third velocity moment. Instead, the distributions can
frequently be best described in terms of two separate solar wind
streams flowing together with somewhat different speeds. For such
cases, the spectra reveal two components, usually incompletely
resolved, which drift relative to one another at approximately the
local Alfven speed. Striking examples of double stream flows that are
relatively well resolved are illustrated in Figure 1, which shows two
E/q spectra obtained about four hours apart with the Los Alamos solar
wind plasma experiment carried on IMP 6. In the spectrum on the left,
the H^+ ions in the higher speed flow have a density of less than 10%
that of the slower component. In this unusual example, the hydrogen
and the helium ions of both of the flows are resolved. Most often, as
in the spectrum on the right, obtained after the flow speed had
decreased by 30-40 km/sec, even though a helium group can be resolved
from the two H^+ groups the two He^++ components are unresolved.
Instead, they appear, as in the right spectrum, as a single component
convecting relative to, and generally faster than, the H^+ ions.

It is interesting to note by inspection of Figure 1, that in four
hours a large change occurred in the density ratios of the faster and
slower streams. The amplitude of the faster H^+ stream in the left
spectrum increased from less than 10% of the slower one to over 100%
in the later spectrum on the right. Also note that the helium
abundance, i.e. the He^++/H^+ ratio, in the two resolved streams in
the left hand spectrum is apparently much higher in the faster, less
dense stream. This is of particular interest, since it is evidence
that bears on the sources of the two streams.

2.3. HEAVY IONS.

Measurements of heavy ion spectra with the solar wind plasma
experiment on Ulysses at those times when kinetic temperatures are low
enough that individual ion species can be resolved in E/q spectra will
add to the body of knowledge already assembled from various
experiments. These experiments include the Los Alamos Vela 3 through 6
electrostatic analyzer instruments, which were the first to resolve
the minor ions, ^(3)He^++ and ^(4)He^+, and the heavy ions of oxygen,
silicon and iron, extending in M/q from O^+7 to Fe^+7. It is expected
that the SWOOPS measurements will supplement those of the Ulysses
SWICS composition experiment which does not have to depend on low
local temperatures of the ions, as does SWOOPS.

Of particular interest for Ulysses, as noted previously, is the
determination of the coronal temperature range over which the Fe
ionization state is established as the high speed solar wind expands
from coronal holes at high latitudes. Simulated heavy ion spectra, two
examples of which are shown in Figure 2, demonstrate the feasibility
of determining coronal hole 'freezing-in' temperatures from E/q
spectral measurements of the Fe ionization state, even under adverse
conditions. These simulations do not include the H^+ ion peak but do
include ions of 14 other elements ranging from He to Ni. The assumed
He^++ local temperatures for the calculations are shown beside those
peaks; local temperatures of the heavier ions were assumed to be
proportional to mass on the left and independent of mass on the right.
From top to bottom in each panel, the ionization states are assumed to
have been established at coronal electron temperatures of 1.5, 2.0,
and 3.0 x 10^6 K. Inspection of the simulations shows that even if in
the high speed streams the heavy ion local temperatures should be
proportional to mass, it should still be possible to determine the
envelope of the iron ion species sufficiently well to establish an
ionization state temperature, or range of temperatures. If the ion
species should be individually resolvable, as in the right panel of
the figure, the determination will be even easier. To resolve the
individual peaks well requires an experiment resolution, [DELTA]E/E,
of about 2.5%.

2.4. SOLAR WIND ELECTRONS.

Solar wind electron velocity distributions measured in the ecliptic
are complex, usually exhibiting a central, low energy 'core'
distribution embedded in a higher energy 'halo' distribution. Most
often, a beamed, unidirectional heat flux component is also present in
the distributions, carrying energy from the hot corona out into the
colder interplanetary space along the interplanetary magnetic field.
Sometimes, the heat flux is observed to be counter-streaming, or
bidirectional, due to unusual coronal/solar wind conditions, such as
the presence of CMEs propagating past the spacecraft (S/C). At other
times, the heat flux electrons disappear from the distributions,
suggesting field merging and disconnection sunward of the S/C. These
heat flux features contribute important information on the global
magnetic topology of the IMF, on the nature of the interplanetary
solar wind, and on its source.

Since virtually no information is available concerning the nature of
electron distributions at higher latitudes, ecliptic results are taken
as the baseline for Ulysses. The left panel of Figure 3, shows cuts
along the magnetic field direction of a typical distribution fitted
with bimaxwellian distributions, using three assumed S/C potentials,
identified in the figure. A S/C potential of 4.5 V gives the best fit
at this time. The functional fit shows the commonly observed low
energy or 'core' component and higher energy but lower density 'halo'
component. It should be mentioned that other functional forms are
sometimes used to fit the halo, but a discussion of them is beyond the
scope of this instrument paper.

As mentioned, a high energy 'heat flux' or 'strahl' component,
strongly beamed along the interplanetary magnetic-field direction, is
often observed. An example, measured with IMP 7, is shown in the right
panel of Figure 3, in which angular cuts at different energies are
given. For these spectra the measurement points were separated by
11.25 [deg.] In the design of the Ulysses experiment, as discussed
later, care was taken that there would be no coverage gaps large
enough for beams like this to be missed. Referring to the figure, the
decreasing angular width with increasing electron energy exhibited by
the beam in this example appears to be well described in terms of a
collisionless expansion of the high energy electrons in which both the
magnetic moment and total energy are conserved.

3. Instrumentation--general.

Electron and ion measurements are made simultaneously with two
completely separate instruments. This provides an important measure of
redundancy since solar wind bulk flow parameters can be derived from
measurements made with either instrument. Each is separately powered,
with its own low-voltage converter, multiple fixed-level analyzer
voltage supply, and CEM high voltage (HV) supply. Each is operated
with an independent microprocessor-based electronics control and data
handling system.

Each instrument makes use of a curved-plate electrostatic analyzer
with a spherical section geometry that is cut off in the form of a
sector. For a sector, particle bending angles from the center of the
entrance aperture to the exit of the plates are the same, independent
of the entry angles. Each analyzer is equipped with a number of CEMs
arranged around the analyzer exit to intercept analyzed particles
which have entered the analyzer within a fan-shaped solid angle of
acceptance. The instruments are oriented on the S/C so that the planes
of symmetry of their acceptance fans are parallel to the spin axis.
Particle arrival directions, then, are measured in a S/C coordinate
system of azimuth angle, scanned by S/C rotation, and polar angle,
determined by which CEMs intercept the particles. Azimuth angle is
measured from the spin angle at which the Sun is included in the
center plane of the acceptance fan and polar angle is measured from
the spin axis, which constantly points toward the Earth. The local
polar and azimuth angles will, of course, be converted to appropriate
frames of reference for analysis.

Spacecraft and mission design parameters were important drivers of the
specific designs of the two instruments. The most critical driver was
the necessity for maintaining the S/C spin axis pointed at the Earth
throughout the mission. As discussed later, this feature greatly
complicates the design of the ion analyzer experiment. Another
important mission feature is the spin period of the S/C, nominally 12
s, which determines the basic timing patterns of experiment cycles. Of
critical importance, the S/C data rates, 1024 bits/s during tracking
and 512 bits/s during store periods, constrain the bit rates that
could be allotted to the plasma experiment, limiting the time and
spatial resolutions that can be achieved. Of the 1024/512 bits/s
totals, the plasma experiment is allotted 160/80 bits/s, divided
112/56 bits/s for ions and 48/24bits/s for electrons. Time resolutions
achieved are discussed in the individual instrument descriptions.

The wide range of heliocentric distances covered during the mission, 1
to 5 AU, was another important driver because each experiment had to
be designed to provide a dynamic range that covers not only the normal
hour-to-hour and day-to-day particle flux intensity variations, but in
addition covers the factor of 25 variation in 1/R^2 that will occur
over the lifetime of the mission. As discussed later, different
solutions for this problem are used for the two instruments.

4. Ion analyzer experiment.

4.1. GENERAL CHARACTERISTICS AND PHYSICAL DESCRIPTION.

Solar wind ion measurements are made with a spherical section
electrostatic analyzer shown schematically in Figure 4. Ions that pass
through the entrance aperture with proper angles are selected in E/q
by applying appropriate voltages across the curved plates. They are
selected in polar angle by the CEMs in which they are detected, and in
azimuth angle by the spin angle at which they are detected, referenced
to the solar direction.

The analyzer, cut off to provide bending angles of 105 [deg.] from the
center of the entrance aperture for all entrance angles, has an
average radius of 100 mm and a plate spacing of 2.84 mm. An angle of
105 [deg.] was selected to give good resolution and slightly
overlapping responses from the 16 CEMs arrayed behind the analyzer
with 5 [deg.] spacings. To avoid backgrounds caused by UV reflections
through the plates to the CEMs, the plates are copper plated and
blackened. The analyzer is mounted on the S/C so that the first CEM
views along the spin axis direction and the sixteenth at 75 [deg.]
from the pole. As discussed later, appropriate sets of the CEMs are
selected for data collection and transmission as the mission
progresses and the angle between the spin axis and the Sun changes.
The intrinsic FWHM energy resolution is about 5%. Because of various
factors, which include the 1-5 AU variation in heliocentric range
during the mission, the spin period of the S/C, the maximum allowable
CEM counting rates, etc., the necessary dynamic range of the
experiment cannot be accommodated simply by varying the length of the
multiplier counting intervals. Hence, a set of seven apertures is
provided on a disk driven by a stepper motor that can be commanded to
select appropriate aperture sizes as the mission progresses. The wheel
includes a blank position that was used during launch and can be used
for inflight background measurements.

The CEMs are powered by a -2400 V to -4000 V HV supply with 16
commandable voltage levels for inflight CEM gain adjustment, as
necessary. Negative high voltage is used to post accelerate the ions
into the CEMs so that they are counted with high efficiency. The CEMs
are operated at average gains of 5-10 x 10^7 electrons per pulse, and
the pulses are amplified with discrete component amplifier-
discriminators with threshold sensitivities of 10^-13 coulomb.
Experiment control and data handling are implemented with an 1802
microprocessor-based logic with read-only (ROM) and random-access
(RAM) memories. Counts, collected in 16-bit serial registers from the
16 CEMS, are coded into 8-bit words for those channels that are
selected as described in the next section and stored in memory for
readout to the S/C data handling and telemetry system.

Analyzer high voltage is provided by a 200-level supply with levels
extending from -15.3 V to -2070 V to cover a range of central energies
extending from 255 eV/q to 34.4 keV/q. Level spacings are 2.5%, and
the levels are used in several different ways, as discussed later. In
some data cycles, the analyzer supply operates in an active range-
adjusting mode which makes more efficient use of the experiment bit
rate. The voltage level at which the solar wind H^+ peak produces the
highest counting rate is determined in each measurement cycle and used
to set the E/q range to cover H^+ and He^++ ions in the next cycle.
The H^+ peak position also determines the measurement ranges for two
other cycles used for measuring heavy ions.

As mentioned previously, a motor-driven disk with seven different
apertures is used to adjust the experiment dynamic range as
heliocentric distance changes. The appropriate aperture for standard
operation at a given heliocentric distance is selected by command. In
a special high- sensitivity mode for heavy ions, discussed in Section
4.5, the largest aperture is selected.

For flexibility during experiment preparation, the experiment design
is modular. Figure 5 shows a photograph of the 4.1 kg instrument which
is composed of a drum-shaped sensor box which is tied to a rectangular
electronics box by means of triangular brackets. The sensor contains
the electrostatic analyzer, CEMs with amplifier-discriminator
circuits, stepper motor, and aperture wheel. The electronics box
contains a low voltage converter, a 200-level analyzer plate voltage
supply, a 16-level CEM HV supply, microprocessor-based electronic
control and data processing systems, and auxiliary timing and
interface circuitry. Interconnections between the packages are made
via cable connectors and tie points in such a way that the units could
be easily joined or separated during ground testing, with no soldering
or major disassembly required.

4.2. SOLAR WIND-SPACECRAFT REFERENCE FRAME.

Because of the Ulysses requirement for keeping the body-fixed high
gain antenna pointed towards the Earth, a particularly difficult
geometry for a solar wind ion experiment had to be accommodated. In
the past, solar wind ion measurements have most often been made on a
spinning S/C with its spin axis inertially fixed, generally
perpendicular to the ecliptic. For such a S/C, 3D measurements of the
ion velocity distributions can be made with an electrostatic analyzer
equipped with eight or nine CEMs with 5 [deg.] spacings. The analyzer
is oriented so that with each spin its fan-shaped acceptance angle is
carried past the Sun with the solar direction centered in the
multiplier array. For such a geometry, the experiment generates an
invariant three-dimensional rectangular array, or 'matrix', of counts
as a function of spin or azimuth angle, CEM number or polar angle, and
analyzer voltage level or E/q. Fitting this data matrix allows a
determination of the various ion parameters sought, such as bulk
velocity, number density, parallel and perpendicular temperatures,
etc.

Contrasting with an inertially oriented S/C is the Earth-oriented
Ulysses geometry. Throughout the mission the angle between the solar
direction, which is approximately the average direction of arrival of
the solar wind, and the Earth-pointing S/C spin axis, changes
continuously; this Sun Aspect Angle (SAA) ranged from above 75 [deg.]
shortly after launch, to values which oscillate near 0 [deg.] as the
S/C nears Jupiter. As the S/C passes over the solar poles, the SAA
will range up to 25 [deg.] during the south polar passage, and up to
30 [deg.] for the north passage.

Figure 6 gives a polar representation of the geometry and highlights
the difficulties for a solar wind ion experiment on Ulysses. The
solution to making adequate measurements is based on the fact that
roughly 5 [deg.] spatial resolution is required in both polar angle
and azimuth to make adequate solar wind ion measurements. At times,
even higher resolution would be desirable, but compromises had to be
made between various factors including experiment complexity, time
resolution, and bit rate usage. In this representation the pole
represents the spin axis at 0 [deg.]; this axis points toward the
Earth constantly. Concentric circles at 5 [deg.] intervals represent
the arcs swept out during each revolution of the S/C by the 16 CEMs
located as shown in Figure 4 and depicted in Figure 6 by the larger
dots aligned vertically upwards from the spin axis.

The basic operating cycle makes use of the S/C Sun sensor and spin
clock to divide each revolution into 64 parts or sectors depicted in
Figure 6 by the lines radiating from the spin axis. As shown in the
next section, a basic measuring pattern is implemented by using the
S/C Sun clock sector pulses and the experiment time clock. During a
spin, the numbers of counts accumulated in 35-ms intervals from each
of the 16 CEMs are recorded at each energy level of a 4-level sequence
that is initiated at every sector. Thus, every intersection of a
circle with a radial line in the figure shows where a 4-level
measurement sequence is initiated. There are a total of 1024
intersections or measurement points every S/C revolution. Since the
accumulated counts at many of these points are zero, because they are
taken with SWOOPS facing away from the solar wind beam, the efficiency
of use of the bit rate allotment could be improved. The sectors are
timed from the Sun pulse so that the Sun always falls at sector 32.5
and a subset of relevant data points within a range of +/- 20 [deg.]
of that central meridian, marked with the smaller dots, was selected
from which data can be chosen for storage and transmission to Earth. A
range of +/- 20 [deg.] was chosen because the solar wind direction
almost always falls inside that range. This selection reduced the
number of data points that have to be covered from 1024 to 185 for
SAAs falling between 0 [deg.] and 60 [deg.].

Still, if the most efficient use is to be made of the allotted data
rate, i.e. if the highest experiment time resolution is to be achieved
within the bit rate allotment, it is desirable to limit further the
number of data samples to be transmitted. This is achieved by dividing
the 185 intersection points into 11 sets, or 'matrices', of 79 points
each, centered on CEM positions from 2 to 12. Figure 7 shows four of
the eleven matrices (MX[n]), MX[0], MX[3], MX[6], and MX[10]. MX[0] is
designed for those times when the solar direction, SAA, lies between 0
[deg.] and 7.5 [deg.] of the spin axis, MX[3] for SAAs between 17.5
[deg.] and 22.5 [deg.], MX[6] for 32.5 [deg.] to 37.5 [deg.], and
MX[10] for SAAs starting at 52.5 [deg.] and extending outward. These
matrix arrays are implemented in logic data processing by selecting
data output from appropriate CEMs and sector angles for storage and
transmission. The matrices are stored in read-only memories (ROMs) in
the electronics logic system, and are available for call-up by command
as the mission progresses and SAA changes.

It is interesting to note in Figure 7 that the form of the data matrix
changes from a nearly polar coordinate format for MX[0] to almost a
rectangular coordinate format for MX[10] A different data analysis
program is required for each of these 11 matrices, greatly
complicating data reduction and placing severe limitations on the
sophistication of analysis algorithms that could be implemented for
the Common Data File. Finally, it should again be noted that, as
described in the next subsection, each of the data points discussed
above actually represents four different measurements at four
different energy levels (and four slightly different angles). Thus,
each data matrix contains 4 x 79 = 316 data points, and a complete set
of measurements acquired in 10 spins contains 3160 data points.

4.3. STANDARD SOLAR WIND (SW) SEQUENCE SEARCH (S) AND TRACK (T)
CYCLES.

The SWOOPS normal SW sequence of operation includes three types of
operating cycle; two of these cover the H^+ - He^++ E/q range, and
another covers the heavier ions of oxygen, silicon, and iron. These
cycles, to be described in detail below, are (1) the Search or S-
cycle, which periodically updates the H^+ peak position and at the
same time provides data from a wider range of E/q, (2) the Track or T-
cycle, which brackets the H^+ and He^++ peaks with an E/q range that
is continuously adjusted up or down for solar wind speed variations,
and (3) a Heavy Ion or HI-cycle, which is discussed in the next
section. Another more sensitive cycle for heavy ions, the H-cycle,
discussed in Section 4.5, is run infrequently on command. Exact
details of these cycles depend on whether the S/C is actively
transmitting or in the data storage mode.

When the S/C is being tracked, the SW sequence consists of a leading
S-cycle to determine the proton peak position, followed by eight track
cycles, during which the E/q range is continually updated. When the
S/C is in the store mode, the sequence order is changed to an S-cycle
followed by four T-cycles. As described later, subcycles of the HI-
cycle are interspersed with the T-cycles, so that at the end of a
sequence, one complete HI-cycle has been obtained.

Both the S- and the T-cycles acquire identical 3160byte sets of data
in 10 spins of the S/C, independently of whether the S/C is being
tracked or is in the store mode. However, the data acquired in 10
spins requires 20 spins, or 4 min, to transmit when the S/C is being
tracked, and 40 spins, or 8 min, to transfer to the tape recorder when
the S/C is in the store mode. With little penalty in overall time,
this additional time is utilized to acquire low bit rate HI-cycle
data, either when the S/C is being tracked or is in store mode.

An almost identical pattern of stepping through 40 levels of the 200
level analyzer voltage supply, four levels at a time each spin, is
used for the S- and T-cycles. The major difference between S- and T-
cycles is the level spacings. Figure 8 shows the voltage stepping
pattern for a T-cycle. In the defined E/q range, every second level is
used, so E/q spacings are 5%. At every sector edge during a spin, a 4-
level sequence of analyzer voltage stepping is repeated while counts
are accumulated at every level into 16-bit registers from all 16 CEMs
in 35-ms counting intervals. Depending on the selected matrix of
coverage, discussed in the previous section, counts from appropriate
CEM registers at selected sectors are compressed to 8bit words in the
microprocessor-based logic, stored in experiment memory, and later
shifted to the S/C for transmission or to the tape recorder. In the
first spin of a T-cycle, counts are accumulated beginning at the
lowest level of the pattern; this level is continuously adjusted up
and down for solar wind speed variations by tracking the proton peak.
The level number, L[max], at which the maximum count (proton peak) is
recorded is determined in each T-cycle and used to establish the first
level of the succeeding cycle in such a way that the cycles alternate
using the odd, O, and even, E, levels of the supply. When conditions
are stable, the complementary sets of data can be interleaved to give
level spacings of 2.5%, the highest resolution available.

The T-cycle E/q range brackets the solar wind ions as illustrated in
the Figure 8, which shows two spectra of counts vs. E/q plotted
vertically at the side of the figure. One of these spectra is typical
of times when local temperatures are low and the two major solar wind
species are very well resolved; the other is typical of times when the
H^+ and He^++ ion peaks are warm and not so well resolved. The E/q
range of the T-cycle is designed to accommodate such variations as
well as those cases when the He^++ ions are travelling faster than the
protons by approximately the Alfven speed.

The S-cycle pattern is the same as the T-pattern, except that it uses
every fourth level of the analyzer supply, instead of every second
level as in the T-cycle. The S-cycle levels are a fixed (F) set which
always begins at the lowest level, level 1, of the supply on the first
spin, and ends at level 157 on the 10th spin. With 10% level spacing,
the 40 levels of the S-cycle cover central E/q values from 255 eV/q to
11.9 keV/q, or taking the FWHM response widths of the analyzer into
account, the full range is 248 eV/q to 12.2 keV/q. Again, at every
sector edge during a spin, a 4-level sequence of analyzer voltage
stepping is repeated, while counts are accumulated in 35-ms counting
intervals at each level. As mentioned, when the S/C is transmitting,
the basic SW sequence begins with a 20-spin Search or S-cycle, which
is actually composed of two subcycles. In the first 10 spins, a 40-
level cycle using every fourth step is covered and the proton peak
position, L[max], is determined with 10% resolution. This 10-spin
subcycle is followed by a second one, which, like the T-cycle, uses
every second level to obtain a refined L[max*] to 5% accuracy. This
L[max*] is used to determine the levels for the first T-cycle of the
sequence and retained for use in all of the HI subcycles throughout
the sequence. The wider range S-cycle data from the first subcycle are
transmitted, but those from the second subcycle are not because of
data rate limitations.

4.4. HEAVY ION HI-CYCLE.

Routine HI-subcycles for heavy ions are run during the extra 10-spin
(transmitting) and 30-spin (store) periods that must follow T-cycles
to allow time for data to be shifted out for transmission or tape
recording. Data from each HI-subcycle are combined with and
telemetered with the T-cycle data that precede the HI-subcycle. The
full HI-cycle is formed by combining eight HI subcycles, all of which
are based on the E/q level of the proton peak, L[max*] that was
determined in the second S subcycle at the beginning of the sequence.
That L[max*] position establishes a set of 64 levels separated by
2.5%, designed to cover heavy ions ranging from O^7+ with M/q = 2.29
to Fe^6+ with M/q = 9.3. Each individual subcycle covers eight energy
levels. To illustrate the sequence, the subcycles are designated '1'
for levels 1-8, '2' for levels 9-16, '3' for levels 17-24, etc. A
subcycle consists of eight spins during which the designated analyzer
voltage levels step up once per spin. Counts from the individual CEMs
are integrated during each spin over the designated sector range and
stored for shifting to the S/C according to the commanded data matrix.
Each subcycle generates up to 72 bytes of data. At the end of a
sequence of eight HI subcycles, the entire 64-level HI-cycle heavy ion
spectrum, integrated over spin angle, has been accumulated.

During S/C tracking periods, one HI-subcycle is run with each T-cycle,
with subcycles identified with a number between 1 and 8. As previously
noted, there are two types of T-cycle, identified as E or O that use
even and odd levels alternately. The SW sequence of cycles, then,
including an initial search cycle, can be written as

S/C-transmitting SW Sequence: S - O[1] - E[2] - O[3] - E[4] - O[5] -
E[6] - O[7] - E[8].

This sequence produces an S-cycle matrix of data, eight T-cycle
matrices, and a 64-level HI spectrum of data. While the S/C is
transmitting, the sequence repeats over and over, with occasional
interruptions when the higher sensitivity heavy ion H-cycle, to be
described in the next section, is commanded to run one cycle. The
nominal time for a complete S/C-transmitting SW sequence is 36 min.

During S/C data store periods when the data rate is reduced to half,
instead of 10 additional spins being required to shift out data from
each cycle, 30 additional spins are required. This extra time is used
to fit in two HI-subcycles with each E and O T-cycle. The SW sequence
of cycles for store periods is then

S/C-store SW Sequence: S - O[12] - E[34] - O[56] - E[78].

This 40-min S/C-store mode SW sequence, like the S/C-transmitting SW
sequence, contains an S-cycle and a 64-level HI-cycle spectrum, but
unlike the S/C-transmitting sequence, it contains only four T-cycles
instead of eight.

4.5. HEAVY ION H-CYCLE.

The heavy ion, H-cycle, which operates on command, places SWOOPS in
its most sensitive configuration by stepping the aperture wheel to its
largest opening. An E/q range of levels that avoids the proton peak
position is used to avoid running the multipliers at excessive
counting rates. This range is established using the latest proton peak
position, L[max], determined in the last T-cycle before the H-cycle
begins. The cycle is similar to the HI-cycle, except that after
stepping to the largest entrance aperture, the 64 E/q levels that
cover the expected positions of O^7+ to Fe^6+ are run in eight groups
of eight-spin subcycles in succession without interruption, one level
each spin. Instead of integrating counts over the entire spin range
designated by the commanded data matrix, as in the HI-cycle, in the H-
cycle counts are integrated between the defined sector edges. Thus,
the H-cycle provides the best heavy ion data, since it is run not only
in the most sensitive experiment configuration, but it also provides
angular resolution in azimuth. The duration of the cycle is 12.8 min.
The number of data bytes produced in a 64-spin H-cycle is 64 levels x
79 matrix points, or 5056, in eight separate groups, each preceded
with sync identification. Because the cycle takes a relatively long
time, and because it is desirable to limit the frequency of stepper
motor operation, the H-cycle is commanded to run only once or twice
per day.

4.6. ION MEASUREMENT ACCUMULATION AND REPETITION TIMES.

The four measurement cycles for ions that produce data matrices for
scientific data analysis are: (1) the Search, S-cycle, (2) the Track,
T- cycle, which uses alternating even, E, and odd, O, analyzer levels,
(3) the Heavy Ion, HI-cycle, formed from HI-subcycles 1-8 obtained
during an SW sequence, and (4) the Heavy ion, H-cycle, which uses the
largest aperture.

As described previously, these cycles are combined together into
sequences that differ somewhat for times of S/C tracking and those of
data storage. The cycle accumulation and repetition times for S/C
tracking and storage times, along with other characteristics, are
given in Table 1.

4.7. ION ANALYZER RESPONSE AND CALIBRATION.

The angular and E/q response functions for the individual CEM
positions were obtained with pre-flight laboratory calibrations in the
Los Alamos plasma analyzer calibration facility. The analyzer was
mounted on an externally controlled two-axis table within a
calibration chamber with the instrument entrance aperture centered in
a beam from a 1-30 keV duoplasmatron ion source with a monitored flux
intensity and fixed energy. The calibration was performed by setting
the instrument at various orientations with respect to the particle
beam and collecting files of the fraction of the beam transmitted as a
function of plate voltage. By covering angle space with suitable
resolution, a three-dimensional matrix of transmission in polar angle,
azimuth, and E/q space was built up for each of the 16 CEM locations.
This collection of 16 matrices, combined together, constitutes a
matrix that fully defines the instrument's response function. Because
the number of individual measurements necessary for such a
comprehensive calibration of an instrument having 16 independent
sensors and various sizes of entrance apertures is so large, it was
not possible to do the calibrations manually. Instead, they were done
with a specially developed, computer-augmented system that
automatically programmed the measurements, gathered and displayed the
data, and recorded it for further analysis in a large computer. The
total number of information bits contained in the ion instrument
calibration is nearly 10^8. These calibrations are used to convert the
matrices of data obtained in flight into meaningful parameters such as
density, flow speed and direction, and components of the temperature.

The calibrated polar angle responses of the 16 CEMs are shown in
Figure 9. The responses were obtained by setting the analyzer for
maximum response with energy and azimuth angle and then sweeping
through polar angle, keeping the particle energy and azimuth constant.
This corresponds to one column taken through the full response matrix
parallel to the polar angle axis.

The FWHM values of the polar angle responses of the various CEMs are
~4.7 [deg.]. The average center-to-center spacing of the CEMs is 5.2
[deg.], in good agreement with the 5 [deg.] design spacing. Cuts taken
through the full response matrix parallel to the azimuth angle axis
show the azimuth FWHM values of the various detectors ranging from 3.1
[deg.] in the central locations to 4.5 [deg.] at the polar angle
extremes. Such a broadening of azimuth response with polar angle is
expected on the basis of mathematical analysis. The broadening, taken
together with the observed reduction of transmission with polar angle
(Fig. 9), results in similar total integrated responses at each
location.

In the final direction through the response matrix, parallel to the
E/q axis, FWHM energy resolutions of ~4.8% are found, in good
agreement with the expected values. The complete calibrated response,
measured in great detail but not shown here, is used for analyzing
flight data.

5. Electron analyzer experiment.

5.1. GENERAL CHARACTERISTICS AND PHYSICAL DESCRIPTION.

Solar wind electron measurements are made with a spherical section
electrostatic analyzer, shown schematically in Figure 10. Electrons
that pass through the entrance aperture with proper angles are
selected in energy with appropriate voltages applied across the curved
plates. They are selected in polar angle by the seven CEMs in which
they are detected, and in azimuth by the spin angle at which they are
detected, referenced to the solar direction.

As shown in the right view, the FWHM acceptance angle of the analyzer
in azimuth depends on polar angle; at 0 [deg.] it is 9.1 [deg.] and at
73 [deg.] it is 28.7 [deg.] The analyzer is mounted in the S/C with
its fan-shaped acceptance solid angle oriented so that the plane of
symmetry of the fan is parallel to the spin axis and the CEM at the
defined 0 [deg.] position views perpendicular to the spin axis.

The 120 [deg.] analyzer has an average radius of 41.9 mm and plate
spacing of 3.5 mm. The intrinsic FWHM energy resolution is ~13%.
Unlike the ion instrument, which requires a variable aperture size, it
is possible to accommodate the dynamic-range requirements of the
electron instrument, imposed by the 1-5 AU heliocentric range
variation, by simply changing the duration of the counting interval by
command. This easier solution, compared to that for the SWOOPS ion
experiment, is due principally to the fact that electron distributions
are comparatively broad, so the counts are spread over all of the
CEMs, energy levels, and azimuth angles, instead of being concentrated
in a tight beam in angle and energy, as is the case with the ions.

The seven CEMs are powered with a +2400 V to +4000 V HV supply with 16
commandable levels for inflight CEM gain adjustment. Positive high
voltage is used in this case, with the CEM funnels biased at +200 V to
provide post acceleration of the lowest energy electrons so as to
count them with high efficiency. The CEMs are operated with gains of
5-10 x 10^7 electrons per pulse. Discrete component amplifiers and
discriminators with threshold sensitivities of 10^-13 C are used to
amplify the pulses. Normalized pulses are counted in 16-bit serial
registers, coded into 7-bit words, and stored in memory in an 8-bit
format for readout to the S/C data handling, storage, and telemetry
system.

The analyzer plate voltage, which is applied to the inner plate, is
provided by a 30-level supply with levels extending from +0.19V to
+180V, giving a central energy range of 0.86 eV to 814 eV, or a total
range of 0.81 eV to 862 eV, when account is taken of the response
width of the analyzer. Standard electron measurement cycles use one of
two sets of 20 energy levels that can be selected by command. The
central energies of one set extend from 0.86 eV to 429 eV, and the
other from 1.7 eV to 814 eV. In another commandable mode, the two sets
of energy levels are used in an alternating sequence. Backgrounds
caused by UV scattering through the plates and by photoelectron and
secondary electron production in the analyzer plates themselves can
contaminate the data to greater or lesser extents, depending on
design. The analyzer plates for the electron instrument are grooved,
in addition to being blackened, to keep the contamination level low.
Grooving was not necessary for ions because of the tighter geometry.
Additionally, a special mode of operation is provided to measure the
background by applying a +9.7 V voltage source to the outer, normally
grounded plate. In conjunction with this, there are special, closely
spaced analyzer voltage levels for the inner plate, beginning just
above the +9.7 V level applied to the outer plate. In a special
'photoelectron' (Ph) cycle that can be operated periodically, the
outer plate voltage is turned on and the Ph levels of the analyzer
supply exercised. In this configuration, all electrons entering the
analyzer from outside are accelerated and thus have energies >=9.7 eV,
while background photoelectrons and secondary electrons produced
within the analyzer itself with energies generally below 9.7 eV will
be unaffected. Thus, the numbers and energy spectra of these
background electrons generated within the plates are determined for
subtraction from the data.

For flexibility, the experiment design is again modular, as shown in
the Figure 11 photograph. The 2.6 kg package is mounted on the S/C
equipment platform inside the S/C body with the entrance aperture
exposed through holes in the side panel and a local thermal control
blanket. The drum-shaped sensor package which contains the analyzer
plates, CEMs, and associated amplifier-discriminator circuits is
mounted on a chassis that is an integral part of a box-shaped
electronics package. The sensor package is easily mounted or removed
from the electronics chassis. The electronics box contains
microprocessor-based logic cards, low voltage converter, analyzer
plate voltage supplies, CEM HV supply, timing circuits, and interface
circuits. Interconnections between sensor and electronics packages are
made through a cable connector and tie points designed so that during
the development and testing stages the units could be easily joined or
separated with neither soldering or major disassembly required.

5.2. SOLAR WIND - SPACE CRAFT REFERENCE FRAME.

For measurements of the broad distributions of solar wind electrons,
no special problems are introduced by the Earth-oriented configuration
of the S/C, unlike the case for measurements of the narrow, anti-solar
directed solar-wind ion beam. Referring again to Figure 10, the
analyzer is mounted on the S/C with the CEM at 0 [deg.] viewing along
the S/C equatorial plane. The other CEMs view at central polar angles
of +/- 21 [deg.], +/- 42 [deg.] and +/- 63 [deg.] with respect to the
equatorial plane. A full polar angle range of over +/- 73 [deg.] is
covered without gaps, because, as will be shown later, the CEMs have
overlapping responses. Because of the broad nature of electron
velocity distributions, measurements must be made covering as much of
the 4[pi] sr unit sphere as possible, independent of the measuring
reference frame. Therefore, in the case of the Earth-oriented Ulysses
frame, all that is required after determining the electron velocity
distribution in the spacecraft reference frame is to transform it into
a universal solar-oriented reference frame.

5.3. BASIC OPERATING CYCLE OF THE ELECTRON SPECTROMETER.

Two-dimensional as well as three-dimensional measurements are made to
avoid the prohibitively long repetition times that transmission of 3D
measurements alone would entail. Even so, repetition times are
unfortunately long. The basic operating cycle of the electron analyzer
is illustrated in Figure 12. It is used to generate four different
science data cycles. These cycles produce energy/angle matrices of
counts with sufficient resolution that various desired parameters of
the electron distribution, such as number density, temperature,
distribution anisotropies, etc. can be accurately calculated. In the
cycle shown in the figure, two analyzer voltage levels with 38%
spacing are alternated 128 times during each spin. In the following
spin, the next pair of levels is alternated. At the end of 10 spins,
20 electron energy levels have been sampled at two interleaved sets of
64 spin angles. Counts are taken from all seven CEMs every time that
the voltage switches and are accumulated in 16-bit serial registers.
Fixed counting times ranging from 4 to 64 ms, are selected by command.
The S/C spin clock is used to divide the spin into 128 intervals, each
spanning 2.8125 [deg.] and a nominal time of 93.75 ms. Near 1 AU, 4-ms
intervals were long enough to provide statistically accurate counts
without danger of counter spills. As heliocentric distance increases
and counting rates diminish, longer counting intervals are selected to
maintain statistical accuracy.

The main reason for using alternating energy levels, as shown in
Figure 12, rather than changing the level once each spin is to acquire
the needed 20 levels of data in 10 spins rather than 20, so that the
data accumulation and repetition times do not become excessively long.
Because the electron beams, or 'strahl', that are found in high-speed
solar wind can be quite narrow, the frequency of level alternation is
kept high and counts sampling is done in such a way that a beam cannot
be missed.

There is one exception to the alternating voltage pattern of
operation. In a cycle designed to determine the gain conditions of the
seven CEMs, the analyzer voltage is held constant while data are taken
over 10 spins at a fixed electron energy. Five CEM HV levels centered
on the presently commanded operating level, and two different
amplifier threshold sensitivity levels, are utilized during the 10-
spin cycle to determine the CEM gain conditions. If the cycle results
indicate a need for higher gain, the CEM HV supply can be commanded to
a higher voltage level. A similar gain calibration cycle is used to
monitor the gains of the ion spectrometer CEMs.

5.4. SCIENCE DATA CYCLES.

There are four science data cycles called 2S, 3R, 3P, and 2P. The
numbers 2 and 3 indicate the number of dimensions of the measurements;
the letters have no special significance. The 2S-cycle gives the
highest resolution in azimuth, which can be particularly useful for
resolving very narrow beams. It is a two-dimensional cycle that
operates as shown graphically in Figure 13, where the alternating
level pattern for levels L(n) and L(n+1) is shown. Considering level
L(n) only, counts are accumulated from each CEM in the commanded
counting interval every time that the voltage switches to L(n). At the
end of each counting interval, the counts from the seven CEMs are
summed together, divided by 2, compressed to a 7-bit word, and stored
in the experiment memory until shifted out to the S/C data-handling
system. The same operations are done on the counts at L(n+1), of
course, and for all other levels during the 10-spin cycle. At the end
of the cycle a data set of counts at 64 angles and 20 energy levels
containing 1280 7-bit data words (1120 bytes), has been accumulated.
Ten bytes of sync and status information are included with each cycle
of data. When the S/C is being tracked, the electron experiment bit
rate is 6 bytes/s, so that a 2S cycle of data is shifted out in 3.14
min. In data storage periods at half the bit rate, the time is 6.28
min.

Although it would be very desirable to have a 3D cycle in which counts
from all seven CEMS at 64 angles per spin are recorded and
transmitted, the cycle readout time becomes prohibitively long. As a
compromise, the 3R cycle (Fig. 13) generates data from each of the
seven CEMS at 32 angles per spin. This is done by accumulating counts
at level L(n) in pairs of counting intervals, adding them together,
compressing to 7-bits, and storing in experiment memory for readout.
At the end of the 10-spin 3R-cycle, counts samples have been
accumulated from seven CEMS at 32 spin angles and 20 energy levels to
give a total of 4480 7-bit words (3920 bytes). To shift this data set,
including status and sync words, to the S/C data-handling system takes
10.92 min when the S/C is being tracked and 21.83 min when it is
recording data.

In the 2P- and 3P-cycles, counts at level L(n) are accumulated in
groups of four counting intervals as shown in the figure. For the 2P-
cycle, the four-interval counts from each of the seven CEMS are summed
and divided by 8 before compressing and storing. In the 3P-cycle,
four-interval counts from each of the seven CEMS are compressed and
stored. Thus, both cycles produce counts at 16 spin angles and 20
energy levels. For 2P, the total number of 7-bit words produced is
320, requiring 0.81 min or 1.61 min for readout. For 3P, 2240 7-bit
words are generated, requiring 5.47 min or 10.94 min for readout.
Various characteristics of these four science data cycles are
summarized in Table 2.

Unlike the ion experiment which has just one standard operating mode,
the electron experiment has two, selectable by command. One called RS,
is designed to give higher spatial resolution; the other, called P,
gives higher time resolution with less spatial resolution. The modes
consist of three cycles in a sequence which repeats. They are listed
in Table 3, along with pertinent times.

5.5. ELECTRON ANALYZER RESPONSE AND CALIBRATIONS.

The angle and energy response functions for each of the individual
CEMS behind the analyzer must be known accurately to convert the data
sets into useful electron parameters. The electron analyzer was
calibrated in the Los Alamos plasma instrument calibration facility in
an electron beam with fixed energy, in the same manner that ion
instrument was calibrated with ions. The polar-angle response results,
shown in Figure 14, were obtained with a single sweep through polar
angle with beam energy and azimuth angle set at values giving maximum
response. Inspection of the figure shows that the geometry of the
experiment provides overlapping response ranges, as desired to prevent
measurement gaps in which a narrow electron beam, or 'strahl', might
be lost or poorly resolved. With contiguous coverage in azimuth as the
S/C spins, this polar angle coverage results in over 95% coverage of
the 4[pi] sr unit sphere.

Much more extensive response functions for each CEM location were, of
course, determined from the laboratory calibrations and are used in
analyzing the data. The FWHMs of the polar angle responses of the
individual CEMs are ~19.5 [deg.] and their calibrated locations are
their design locations to within 2.5%. Cuts through the full response
matrix parallel to the azimuth axis give FWHMs of the azimuth
responses ranging from 9.1 [deg.] in the center to 28.7 [deg.] at the
highest polar angles. Although the polar angle sweep shown in Figure
14 shows a large reduction in amplitude at the high polar angles, when
the response broadening in azimuth is taken into account, the
integrated responses of the CEMs from the center to the extreme
locations are as expected. Cuts parallel to the energy axis show FWHMs
for the energy responses of 11-12%.

6. Early flight results.

Following the launch of Ulysses aboard the shuttle Discovery on 6
October 1990, and its injection into an interplanetary orbit toward
Jupiter by the upper rocket stages, mission and S/C constraints
required a substantial waiting period before the SWOOPS electron and
ion experiment packages could be activated. These constraints included
S/C thermal considerations due to the early orientation of the S/C
spin axis with the Sun, and the requirement for a long pump-out period
to achieve clean, high vacuum conditions inside the SWOOPS sensor
packages before operating the CEMs under high voltage. After some
preliminary activities on 16 November, both packages were switched on
successfully on 17 November, six weeks after launch. In this section,
we present some early results to illustrate the types of data that are
being received from Ulysses and to show the nature of the data
products that are available from SWOOPS for carrying out the research
aims of Ulysses during the three phases of the mission: the in-
ecliptic cruise phase, the Jovian encounter, and the out-of-the-
ecliptic cruise to high solar latitudes with passages over the south
and north poles of the Sun.

6.1. SOLAR WIND IONS - E/q SPECTRA AND STACKED PLOTS.

As discussed in Sections 4.1 through 4.3, solar wind ion data from
search or track cycles are obtained in blocks or 'matrices' of counts
that cover selected polar angles (CEMs), azimuth angles (spin angle)
and E/q (analyzer voltage) levels. Forty E/q levels with 5% spacings
are selected from the 200-level supply to span a range that is
dynamically adjusted up and down to suitably bracket the ambient
proton and helium ion peaks as their speed (energy/charge) varies. The
most basic form of solar wind ion data is a matrix of counts covering
79 CEM-azimuth positions at each of the 40 E/q levels. A subset of
such a matrix of counts, selected for an azimuth that gives a cut
through the proton and helium ion peaks, is shown graphically in
Figure 15. The spectrum shows counts collected from the CEMs at a
single azimuth during 35-ms counting intervals as the analyzer voltage
is stepped up, four levels per spin. Data from the individual CEMs are
identified with different line types. The local temperatures of the
protons and helium ions are low enough at this time that the peaks are
cleanly resolved. Here, counts from the nine CEMs in the chosen matrix
are plotted as a function of E/q step number, which is convertible
into E/q values. To identify those step positions where the count is
zero, points are plotted half a decade below the one-count level. The
data set in Figure 15 was obtained soon after SWOOPS was turned on,
when the matrix in use collected data from a set of nine contiguous
CEMs.

Another form of solar wind ion data presentation that has been useful
in the past and is being used to display Ulysses data, gives a
pictorial representation of the flow conditions, as shown in Figure
16. This form, known as 'stacked plots', stacks E/q spectra vertically
in a closely spaced time sequence, from top to bottom. Individual
spectra are formed by integrating counts over CEMs and azimuth at each
E/q level. Normally, the spectra show a prominent peak due to the
protons with a second peak at twice the E/q that is due to doubly-
ionized helium. Stacked plots present the data in a compressed form
that makes such features as interplanetary shocks and other interfaces
easy to recognize by sudden shifts in peak positions, broadening or
thinning of spectral widths, changes in the relative sizes of peaks,
etc. For example, in those rare cases when singly ionized helium ions,
He^+, are present in substantial quantities, their presence is
immediately obvious in a stacked plot by the unusual appearance of a
third ion species peak at an E/q position four times that of the
proton peak.

Figure 16 is a stacked plot from SWOOPS data for a 12-hour period from
1200 UT to 2400 UT on 23 November 1990. Inspection of the plot shows
the expected proton and He^++ peaks during a half day when the speed
is relatively constant, decreasing slightly from start to finish. The
periodic, darker spectra are search-cycle data which cover a wider E/q
range and reestablish the proton peak position every ninth cycle. This
was a time of relatively high local ion temperatures, as shown by the
somewhat broad proton and helium peaks that are only moderately well
resolved. Although a good example showing an interplanetary shock or
other pronounced feature is not yet available, the example given here
shows a few features of interest and demonstrates the nature of the
stacked plot presentation.

At the top of the figure, the first couple of spectra show a rapid,
but small decrease in speed by the downward shift in the proton peak
E/q position. At several other times there are some small, fast
changes in speed and/or temperature which show up as extra white
between spectra, e.g. at about 1530 UT, 1800 UT, and near 2100 UT.
There are other times when the spectra show evidence that two solar
wind streams were flowing together at this time, as discussed in
Section 2.2 and shown in a much more striking example in Figure 1.
Between 1800 UT and 1830 UT, the proton peak shows a 'dimple' which is
evidence for a double stream. Near the end of the day, double stream
shapes are again seen, and the very last spectrum clearly shows that
the proton peak is made up of two overlapping distributions and there
is a suggestion from the width of the helium peak that it also
contains two groups.

6.2. SOLAR WIND IONS - VELOCITY DISTRIBUTION MOMENTS.

Perhaps the most generally useful form in which solar wind plasma data
can be presented is that of plots of velocity distribution moments,
such as density, bulk speed, direction, temperature, etc. This form is
often used for complementary studies which utilize data from other
investigations and for in-depth studies of the underlying physics of
space plasmas. Figure 17 presents ion moments for the second half of
18 November 1990, the first full day of operation of SWOOPS after it
was turned on. The proton density, speed, temperature, and angles of
flow are presented, along with the He^++/H^+ number density ratio,
i.e. the helium abundance. Note that the polar and azimuth flow angles
refer to a S/C coordinate system, where polar angle is determined by
the CEMs from which measurements are taken, and azimuth angle is
determined by the spin angles of the measurements, referenced to the
plane in which the Sun lies. Features in this moment plot will be
related to features in Figure 20 in the last section. Inspection of
Figure 17 identifies a number of features of interest. For example, at
1400 UT, after a period of rising speed and fluctuating direction,
there was a very sudden drop in speed that was not accompanied by
significant changes in density or temperature. However, it was
accompanied by the sudden appearance of a higher abundance of helium
ions that was followed by a fluctuating abundance that reached a
maximum shortly before 1500 UT. The moment data also show that these
changes occurred during a time when there were strong fluctuations in
the proton direction of flow. This feature is most likely a solar wind
discontinuity, such as a tangential discontinuity, but further
analysis is needed, perhaps including data from other investigations,
before the nature of the discontinuity can be stated with confidence.
At about 1545 UT, there were rapid changes in speed coupled with
strong changes of direction, but little change in density and
temperature. Later, at about 1900 UT, there were sudden and
substantial increases in density and temperature, coincident with a
sudden swing in the azimuth direction of flow. Although changes like
these are often associated with interplanetary shocks, in this case,
that interpretation seems weak because the change in speed is so low.
Again, further interpretation is needed to identify the type of this
discontinuity.

There are, of course, other ways to display the ion data that are not
illustrated here. For example, for some studies, contour plots of the
velocity distributions can be very instructive. Such features as the
alignment of parallel and perpendicular components of the temperature
with the magnetic field, and heat flux tails on the distributions are
shown well in the contour format.

6.3. SOLAR WIND HEAVY IONS.

As discussed in Sections 4.4 and 4.5, there are two cycle types
included in the SWOOPS ion experiment for the purpose of gathering
information on the heavy ions beyond He^++, including ions of O^+7 and
O^+6, Si^+9 to Si^+7, and particularly Fe^+12 to Fe^+7. Occasionally,
in CME flows, ions of Fe^+16 may be detected. These HI- and H-cycles
make use of the measured proton peak position to establish the E/q
range in which these ion species would be found. Generally, the HI-
cycle provides rather low numbers of counts that will be mainly useful
during times of long-term stable conditions when the local ion
temperatures are low. A second heavy ion H-cycle, described in Section
4.5, is run on command using the largest aperture, and thus has the
highest sensitivity.

A main objective for including these heavy ion measurements in the
SWOOPS capabilities was to determine the coronal electron temperatures
at which the iron ionization state is established, or 'frozen-in', as
the high speed solar wind expands from the coronal holes. As is
evident from the Figure 2 consideration of this problem, even if the
local temperatures of heavy ions are too high for resolution of
individual ion peaks, it should be possible to define the envelope
shape of the iron ion species under favorable conditions and thus
obtain an estimate of the coronal electron temperatures over which the
ionization state was established.

Figure 18 shows the heavy ion spectrum that was obtained when the H-
cycle was first run on 11 December 1990. Included in the figure are
the positions of the proton and helium ion peaks, and the positions of
various heavy ions that have been identified in earlier measurements
of E/q spectra on Vela and other missions. At this time, local
temperatures of the oxygen and silicon ions at the identified
positions were too high for those species to be resolved. However, the
group of iron ion species, lying beyond the lighter ions, is well
separated from them and, indeed, some of the individual Fe ion peaks
are close to being resolved. The envelope of the Fe lines yields an
estimate, shown in the figure, of the span of coronal electron
temperatures over which the iron ionization state was established, or
'frozen in'.

Analysis shows that the coronal electron temperature over which this
Fe ion state was formed ranged from 1.0 to 1.5 x 10^6 K. Previous
observations have established that this state tends to be broad,
showing that the individual species pair ratios forming the envelope
are established over a range of altitudes in a corona with a
temperature gradient.

6.4. SOLAR WIND ELECTRONS - ENERGY SPECTRA, VELOCITY DISTRIBUTIONS,
AND MOMENTS.

As discussed in Sections 5.3 and 5.4, SWOOPS solar wind electron
measurements are made with a cycle in which two energy levels are
alternated 128 times in a S/C rotation; the level pairs step up each
spin, so that in 10 spins, a full 20-level set of data is obtained.
This set forms a basic matrix of counts at 64 azimuth positions (spin
angle) and 7 polar angles (CEMs), for each of 20 energy levels
(analyzer voltage). From these matrices, a number of two- and three-
dimensional measurement cycles, named 2P, 2S, 3P, and 3R are formed,
to make efficient use of the allotted telemetry rate. Although it
would be desirable to send back all of the data from all seven CEMs
with full angular resolution for each set of data, it is necessary to
reduce the number of azimuth and polar angles for some of the sets as
given in Table 2 to keep repetition times reasonably low.

Figure 19 presents a set of data from a 3P cycle of operation,
selected at a fixed azimuth from a data set measured on 18 November
1990. Counts from the seven CEMs that were acquired at each of the
voltage steps, are plotted. (The voltage step numbers are of course
convertible to energy levels.) This plot shows the dominant features
of solar wind electron data sets. Data taken from the seven
multipliers as the energy is stepped up show a strong group of
ambient, low energy photoelectrons surrounding the S/C with energies
ranging from 0.86 eV to about 5.1 eV. Above the photoelectron
distribution is the core distribution of solar wind electrons,
discussed in Section 2.4, that emerges out of the low energy
photoelectrons as they fall off. At higher energies, beyond the core,
the less dense population of halo electrons is present. Two of the
multipliers also observe a heat flux tail, identified by the elevated
count rates at higher energies.

Not shown here are a few other forms for presenting solar wind
electron data that can be quite useful for specialized studies. These
forms include velocity distributions plotted as shown in Figure 3. For
some studies, showing the velocity distributions in the form of
contour plots is a powerful way to bring out certain important
features of the electrons, e.g. the heat flux and whether it is
unidirectional, bidirectional, or missing. Contour plots can be
particularly useful to illustrate how the electron distributions
evolve with time during an event. Electron moments can be derived from
the velocity distributions so as to display the time evolution of
electron density, bulk flow speed and direction, parallel and
perpendicular temperatures, heat flux, heat flux direction, etc., in a
manner similar to the ion moments shown in Figure 17.

6.5. SOLAR WIND ELECTRONS - COLOR SPECTROGRAMS.

Figure 20 presents a color spectrogram of electron data obtained in
the period 1200 UT to 2400 UT on 18 November 1990. The vertical bars
on the right of the spectrogram show the color codings for the
counting rate levels of the top panel, above, and the rest of the
panels, below. Universal time in hours is shown along the bottom with
some pertinent ephemeris information. Included with the electron data
is a spectrogram in the top panel labelled 'IONS', which shows ion E/q
spectra extending over a total E/q range of 248 eV/q to 12.2 keV/q.
This is the range of the search cycle when the intrinsic energy
resolution of the analyzer is taken into account. Ion moments from the
same time span are those given previously in Figure 17. Periodic light
blue stripes extending from the bottom to the top in the panel, are
spectra from the periodic search cycles. This 40-level cycle, which
runs once every nine cycles in the solar wind sequence of operation,
uses every fourth analyzer level starting at the bottom level of the
200-level supply. Thus, it covers a larger E/q range than the 40-
level, dynamically adjusted track cycles which use every second level.
These T-cycles form the narrower color band which contains the proton
and helium ion distributions (see Sect. 4.3 for further details).

The second panel of Figure 20 shows a spectrogram of the energy
spectra of solar wind electrons, extending from 0.81 eV to 862 eV.
These data were obtained at a time when the two commandable energy
ranges were being alternated, resulting in the dark 'box cars' along
the bottom of the panel. Here, referring to Figure 19, the orange and
red band at low energy is due to the ambient photoelectrons which
produce the highest counting rates. Above the orange band, which first
tapers off to yellow and then green as the photoelectron flux
intensity falls off with increasing energy, lies a yellow band caused
by the broad peak of the core distribution. Above the core
distribution, the intensity of the halo flux falls off more slowly
with increasing energy (see Fig. 19).

Panels 3, 4, 5, and 6 from the top, show angular distributions of the
electrons in the four energy bands identified on the left. The bottom
two panels give spectrograms of the counts distributions in the seven
CEMs in two energy bands. Note that the resolution in those panels is
broader, since there are only seven multipliers.

The features in this spectrogram can be inspected in combination with
the ion moments in Figure 17. Starting before 1300 UT, the solar wind
bulk flow speed began to

rise, in concert with strong swings in the flow directions of the ions
and angular variations in flux intensity maxima of the various energy
bands of electrons, particularly the 212-454 eV band. Approaching 1400
UT, the speed increased from below 400 km/sec to 500km/sec and
flattened off. At 1400 UT, the speed suddenly dropped to below 400
km/sec, coincident with a sudden increase in the helium abundance and
a strong shift in the direction of 212-454 eV electrons. However, this
change was not accompanied by changes in the proton density or
temperature. This event is best interpreted as an interface in the
solar wind, perhaps a tangential discontinuity, but not as an
interplanetary shock. Further study will be required to determine the
exact nature of this interface.

Later, shortly before 1600 UT, there were some fast fluctuations in
the proton speed and direction at a time when the helium abundance had
settled back down from high values to more conventional values. Again,
the proton density and temperature do not change drastically, but the
electron angular distributions show strong changes, mirroring the ion
speed changes. Further study, which might include collaborative use of
data from other Ulysses investigations, may help elucidate this
behavior.

After this episode, the proton speed remained fairly constant for
several hours while the density and temperature gradually fell, until
just past 1900 UT, when both the density and temperature rose
suddenly, along with a sharp change in flow direction. There were also
strong shifts in the direction of the electron distribution maxima in
the 5th and 6th panels of the spectrogram at this time. However, there
was only a very modest increase in the flow speed. The spectrogram
electrons at this time show an increase in the intensity of the color
in all of the panels, which is an indicator of an increase in density
and possibly temperature as well. Since the Figure 17 ion moment data
show a density increase of a factor of two, as also exhibited in the
top panel of the spectrogram by the appearance of the red band at
about 1910 UT, the electron density must have increased also, which
would account, at least in part, for the brighter yellow. Indeed,
electron moment data that are not shown here show that the combined
core/halo density did increase by a factor of two, but the electron
temperature did not increase appreciably across the interface. Thus,
it would seem that this solar wind discontinuity was not due to the
passage of an interplanetary shock. Further study of these data, with
the cooperative inclusion of data from other investigations,
particularly the magnetic field investigation, is required before a
firm interpretation of the nature of this discontinuity can be made
with certainty.

6.6. COMPARISON OF ELECTRON AND ION RESULTS.

A good indication of the accuracy of the SWOOPS plasma measurements,
made with separate instruments for electrons and ions, can be obtained
by comparing the densities and bulk flow speeds determined
independently with the two instruments. As shown in Figure 21, the
electron and ion parameters compare remarkably well. Such agreement
verifies the quality of the separate and independent calibrations of
the two instruments and the integrity of the data analysis techniques.
The small discrepancies are not surprising considering the various
uncertainties inherent in measurements of this nature. One of the most
important of these uncertainties is the distortion of the solar wind
electron velocity distribution caused by a varying spacecraft charge.
This effect can be quite significant for the Ulysses spacecraft which
reaches potentials in excess of 5 V with respect to the ambient plasma
medium in which the spacecraft is immersed. The level of charging is
determined from the shapes of the photoelectron and core/halo electron
distributions and used to correct for the distortion.

The top panel of Figure 21 shows electron and ion densities compared
by plotting the occurrence distribution of N[e]/N[i] in bins of 0.02
for a 1250-spectrum data set obtained between 1.16 and 1.29 AU. The
bottom panel shows the occurrence distribution of V[e]/V[i]. Ion
number densities and speeds for comparison with electron number
densities and flow speeds were obtained by combining proton and helium
ion densities and speeds with appropriate weighting for charge.
Electron measurements used in the analysis are for 3D data sets only;
the analysis includes both core and halo distributions. Photoelectrons
are of course removed in the analysis. Inspection of the figure shows
that on average the electron and ion densities and flow speeds agree
to within about 5%, which is surprisingly good agreement for
independent experiments of this nature. The data show that significant
deviations of 15% or more can occur; such deviations are probably due
principally to inaccuracies in the determination of spacecraft
potential.

Acknowledgements.

The Ulysses Solar Wind Plasma Experiment Coinvestigators and
Associates include J. R. Asbridge (Los Alamos), S. J. Bame^1 (Los
Alamos), A. Barnes (NASA/ARC), W. C. Feldman (Los Alamos), B. E.
Goldstein^2 (JPL), J. T. Gosling (Los Alamos), T. E. Holzer (HAO), D.
J. McComas (Los Alamos), M. Neugebauer (JPL), J. L. Phillips^3 (Los
Alamos), H. Rosenbauer (MPAe), G. L. Siscoe (UCLA), S. T. Suess
(NASA/MSFC), M. F. Thomsen (Los Alamos), and R. D. Zwickl (SEL/NOAA).
We wish to acknowledge the help that has been provided to us by
various members of this group, including participation in the
definition of the original scientific objectives and preparation of
the proposal. This team will participate together and with other
Ulysses investigators on various scientific studies, as the Ulysses
Mission continues.

We wish to thank the technical staffs at Los Alamos and Sandia for the
support that has been provided in preparing the Ulysses Solar Wind
Plasma Experiment. We also wish to acknowledge the guidance and
support provided to us in a very professional manner by the Project
Staffs at ESTEC, including K.-P. Wenzel, ESA Project Scientist, P. J.
Caseley, Experiment Manager, and Derek Eaton, Project Manager, and at
JPL, including E. J. Smith, NASA Project Scientist, T. Tomey,
Experiment Manager, and Willis Meeks, Project Manager. This research
has been done in part by the Los Alamos National Laboratory and the
Sandia National Laboratories, Albuquerque, under the auspices of the
US Department of Energy with financial support provided by NASA, and
in part by the Jet Propulsion Laboratory, California Institute of
Technology, under contract to NASA.

It is beyond the scope of this paper to give a comprehensive reference
list of the many sources from which the investigation objectives,
background, and other details in the paper were drawn. Such a list, if
complete, would occupy more space than is available. However, it
should be acknowledged that some of the material given in this paper
is derived from an earlier description of the SWOOPS investigation
given by S. J. Bame, J. P. Glore, D. J. McComas, K. R. Moore, J. C.
Chavez, T. J. Ellis, G. R. Peterson, J. H. Temple, and F. J. Wymer:
1983, in The International Solar Polar Mission - Its Scientific
Investigations, esa SP-1050, K.-P. Wenzel. R. G. Marsden, B. Battrick,
ed., ESA Scientific and Technical Publications Branch, ESTEC,
Noordwijk, The Netherlands, p. 47. Figures drawn or adapted from that
source are acknowledged in the captions.

^1 Principal investigator
^2 Data analysis team leader, JPL
^3 Data analysis team leader, Los Alamos

TABLE I. Characteristics of ion measurement cycles of the solar wind
ion analyzer.

Cycle      Level      Data    Accumulation time (min)    Repetition time (min)
        spacing (%)   bytes   S/C track    S/C store    S/C track    S/C store
-------------------------------------------------------------------------------

 S        10.0        3160       2.0          2.0         36            40
 T         5.0        3160       2.0          2.0          4             8
HI         2.5         576      29.6         26.6         36            40
H*         2.5        5056      12.8
-------------------------------------------------------------------------------
* H-mode by command during track periods at requested times.


TABLE II. Characteristics of the data cycles of the solar wind
electron analyzer.

        Number  Number  Number    Azimuth        Accumul.   Readout time (min)
       azimuth  polar   energy     angle           time
Cycle   angles  angles  levels   separation       (min)     Track      Store
------------------------------------------------------------------------------

2P       16      1       20    22.500[degrees]     2.0       0.8        1.6
2S       64      1       20     5.625[degrees]     2.0       3.1        6.3
3P       16      7       20    22.500[degrees]     2.0       5.5       10.9
3R       32      7       20    11.250[degrees]     2.0      10.9       21.8
------------------------------------------------------------------------------


TABLE III. Electron experiment commandable modes for spacecraft
tracking and data store times.

                                                    Cycle start times (min)*
                         Data       Sequence
Mode      Sequence       rate      time (min)     First    Second    Third
----------------------------------------------------------------------------

P         3P-2P-2P      Track         7.1          0         2.3       4.7
P         3P-2P-2P      Store        14.2          0         4.7       9.3
RS        3R-2S-2S      Track        17.2          0         5.7      11.3
RS        3R-2S-2S      Store        34.4          0        11.3      22.7
----------------------------------------------------------------------------
* Cycle accumulation times are all ~2.0 min.


FIGURE 1. Two unusual solar wind ion spectra of the common ions, H^+
and He^++, obtained with IMP 6, showing two streams flowing together
with different speeds.

FIGURE 2. Simulated heavy ion spectra, offset from each other, showing
the feasibility of determining Fe ionization state temperatures in
high speed solar wind from coronal holes, using E/q measurement alone.

FIGURE 3. Bi-maxwellian fit to a typical solar wind electron velocity
distribution (left panel), cut through along the magnetic field
direction to show the existence of a low energy or 'core' component
embedded in a higher energy but lower density 'halo' or 'strahl'
component. A third "heat flux" component of electrons flowing out
along the interplanetary field from the hot corona is illustrated on
the right.

FIGURE 4. Ion optics of the SWOOPS ion experiment with a cross-
sectional view on the left and a view from behind on the right. (From
esa SP-1050, p. 57).

FIGURE 5. Photograph of the solar wind ion experiment package. (From
esa SP-1050, p. 59).

FIGURE 6. Polar representation of measurement space for solar wind
ions. (From esa SP-1050, p. 60).

FIGURE 7. Four of the 11 measurement matrices. Each matrix, MX[n]
contains 79 measurement points. (From esa SP-1050, p. 61.)

FIGURE 8. Pattern of operation of the 40 analyzer plate voltage levels
during a 10-spin ion track of T-cycle. Typical H^+ - He^++ spectra to
be covered by the 40 levels are shown plotted vertically on the right
side of the figure. (From esa SP-1050, p. 62).

FIGURE 9. Calibrated polar acceptances of the 16 ion CEMs. (Adapted
from esa SP-1050, p. 65).

FIGURE 10. Electron optics of the SWOOPS electron analyzer. (From esa
SP-1050, p. 67).

FIGURE 11. Photograph of the SWOOPS solar wind electron experiment
package. (From esa SP-1050, p. 69)

FIGURE 12. Analyzer plate voltage level operation during the 10-spin
electron experiment operation cycles. (From esa SP-1050, p. 69)

FIGURE 13. Schematic description of the four electron science data-
gathering cycles: 2S, 3R, 3P, and 2P. (From esa SP-1050, p. 70).

FIGURE 14. Polar angle response of the SWOOPS solar wind electron
experiment. (Adapted from esa SP-1050, p. 72).

FIGURE 15. Solar wind ion E/q spectrum obtained soon after SWOOPS
turn-on.

FIGURE 16. Stacked plot of E/q ion spectra obtained on 23 November
1990.

FIGURE 17. Ion moments for the second half of 18 November 1990.

FIGURE 18. Solar wind heavy ion spectrum measured on 11 December 1990.

FIGURE 19. Solar wind electron spectrum obtained on 18 November 1990.

FIGURE 20. Color spectrogram of the solar wind plasma conditions on 18
November 1990.

FIGURE 21. Comparisons of number densities and flow speeds determined
with the independent electron and ion electrostatic analyzer
experiments.