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Astronomy and Astrophysics Supplement Series, Ulysses Instruments Special
Issue, Vol. 92, No. 2, pp. 267-289, Jan. 1992. Copyright 1992 European
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Astron. Astrophys. Suppl. Ser. 92, 267-289 (1992)
The Solar Wind Ion Composition Spectrometer
G. Gloeckler^1, J. Geiss^2, H. Balsiger^2, P. Bedini^3, J.C. Cain^3,
J. Fischer^2, L.A. Fisk^4, A.B. Galvin^3, F. Gliem^5, D.C. Hamilton^3,
J.V. Hollweg^6, F.M. Ipavich^3, R. Joos^2, S. Livi^7, R. Lundgren^3,
U. Mall^2, J.F. McKenzie^7, K.W. Ogilvie^8, F. Ottens^8, W. Rieck^5,
E.O. Tums^3, R. von Steiger^2, W. Weiss^7 and B. Wilken^7
^1 Department of Physics and IPST, University of Maryland, College
Park, Maryland, USA
^2 Physikalisches Institut, Universitat Bern, Bern, Switzerland
^3 Department of Physics, University of Maryland, College Park,
Maryland, USA
^4 Office of Space Science and Applications, National Aeronautics and
Space Administration, Washington, DC, USA
^5 Institut fur Datenverarbeitende Anlagen, Technische Universitat
Braunschweig, Braunschweig, Germany
^6 Department of Physics, University of New Hampshire, Durham, New
Hampshire, USA
^7 Max-Planck-Institut fur Aeronomie, Katlenburg-Lindau, Germany
^8 National Aeronautics and Space Administration/Goddard Space Flight
Center, Greenbelt, Maryland, USA
Received April 11; accepted September 16, 1991
Abstract. -- The Solar Wind Ion Composition Spectrometer (SWICS) on
Ulysses is designed to determine uniquely the elemental and ionic-
charge composition, and the temperatures and mean speeds of all major
solar-wind ions, from H through Fe, at solar wind speeds ranging from
175 km/s (protons) to 1280 km/s (Fe^8+). The instrument, which covers
an energy per charge range from 0.16 to 59.6 keV/e in ~ 13 min,
combines an electrostatic analyzer with post-acceleration, followed by
a time-of-flight and energy measurement. The measurements made by
SWICS will have an impact on many areas of solar and heliospheric
physics, in particular providing essential and unique information on:
(i) conditions and processes in the region of the corona where the
solar wind is accelerated, (ii) the location of the source regions of
the solar wind in the corona; (iii) coronal heating processes; (iv)
the extent and causes of variations in the composition of the solar
atmosphere; (v) plasma processes in the solar wind; (vi) the
acceleration of energetic particles in the solar wind; (vii) the
thermalization and acceleration of interstellar ions in the solar
wind, and their composition; and (viii) the composition, charge states
and behavior of the plasma in various regions of the Jovian
magnetosphere.
Key words: interplanetary medium -- Jupiter -- artificial satellites
space probes -- sun: solar wind.
1. Introduction.
Knowledge of the solar wind elemental and ionic-charge compositions,
and their variabilities, is essential for our understanding of the
physics of the solar wind, the solar atmosphere, and the Sun itself.
Measurements of its composition and the mean speeds, kinetic
temperatures and charge-state distributions of its major ions under
the varying solar-wind conditions and flows will reveal the conditions
and processes occurring in the regions where it is accelerated. Such
measurements will also provide new information about the elemental
composition of the solar atmosphere and processes that lead to
variations in this composition, and they will enable us to
characterize and study plasma processes occurring in the heliosphere.
The He/H ratio in the solar wind has now been measured for nearly
thirty years and found to be quite variable (e.g. Bame et al. 1977),
ranging from a fraction of a percent to 35%. Electrostatic deflection
analyzer plasma detectors can provide composition measurements of
major heavy-ion species (O, Si, and Fe), but only under favorable
solar-wind conditions (Bame et al. 1979). Foil detectors placed on the
lunar surface during the Apollo 11, 12, and 14-16 missions provided
important measurements of the noble-gas elements in the solar wind and
their isotopes (Geiss 1973). Since the late 1970s a magnetic mass
spectrometer has been operated on board the ISEE-3/ICE spacecraft,
making it possible to study the elemental composition and charge
states of major solar-wind ions under less restrictive solar-wind
conditions (Coplan et al. 1978) (Ogilvie & Vogt 1980).
More recently, a time-of-flight instrument, similar to the one
described in this paper, was flown on the AMPTE spacecraft (Gloeckler
et al. 1985). During the occasional excursions of this spacecraft into
the magnetosheath, the thermalized solar wind gas in that region could
be analyzed, providing for the first time unequivocal information on
carbon and magnesium in the solar wind (Gloeckler & Geiss 1989).
However, important questions concerning the solar wind composition
still remain open, not only at elevated solar latitude, but also in
the ecliptic. Elements like carbon or magnesium have not yet been
thoroughly studied, the information on charge states is largely
confined to oxygen and iron. Systematic measurements of the
composition and charge states in high-speed streams are lacking.
The Ulysses mission presents a truly unique opportunity to investigate
many of the outstanding and fundamental problems of solar-wind physics
using advanced instruments (see Wenzel et al. 1991). The spacecraft's
out- of-ecliptic trajectory will add a new dimension to these studies,
since the three-dimensional properties of the corona and of the solar
wind will be observed for the first time (Geiss & Bochsler 1986). The
capability of the SWICS to measure the elemental and charge state
compositions of ions (Gloeckler et al. 1983) will also provide the
first detailed charge state and species information for the hot Jovian
plasma and its suprathermal component.
A preliminary look at the post-launch data reveals that the Solar Wind
Ion Spectrometer experiment on Ulysses will provide comprehensive
elemental and ionic-charge composition measurements of solar wind ions
from H through Fe, of interstellar ions, and of the Jovian plasma
during Ulysses' Jupiter flyby. The instrument's design, fabrication
and testing was a collaborative effort by five institutes: the
University of Maryland, the University of Bern, the Max-Planck-
Institut fur Aeronomie, the Technical University at Braunschweig, and
the Goddard Space Flight Center. In the following sections we will
outline the scientific objectives of our investigation and describe
the SWICS instrument and its characteristics and response in some
detail.
2. Scientific objectives.
Our investigation on Ulysses will address a number of fundamental
problems. These will include studies of: (a) solar-wind acceleration
and flow, (b) the composition of the solar atmosphere, (c) plasma
processes in the solar wind, (d) interstellar ions, and (e) the
composition of the Jovian plasma. On a pioneering mission such as
Ulysses and with an advanced instrument such as the SWICS, we will
undoubtedly also discover solar, heliospheric and Jovian phenomena
whose existence cannot be anticipated from our present perspective.
2.1. STUDIES OF SOLAR PHENOMENA.
A major scientific objective of the Ulysses mission is to undertake a
systematic study of the flow of the solar wind over the poles of the
Sun. At such high latitudes, the solar wind is expected to be less
complex and therefore easier to understand than other solar-wind flows
in the heliosphere. The flow, originating from a semi-steady coronal
hole at each pole, is expected to be reasonably constant in speed and
unencumbered by the stream-stream interactions that complicate the
flow at lower latitudes. By making detailed measurements of the
elemental and ionic-charge composition of the solar wind, and of the
flow properties of solar-wind ions over the poles as well as at other
heliographic latitudes, we will obtain essential information on the
conditions characterizing, and the physical processes operating in the
region of the corona where most of the solar-wind acceleration occurs.
The solar-wind acceleration region lies relatively high in the corona
(a few to several solar radii from the Sun's center), where the
density is very low (especially in coronal holes). Spectroscopic
measurements at these high solar altitudes are very difficult to
obtain. In contrast, an abundance of information regarding the
acceleration region is available from measurements of solar-wind ions.
Indeed, it is quite convenient that the very particles whose
acceleration and flow are being studied carry the needed information
with them in their composition and behavior.
More specifically, our measurements with the SWICS will contribute to
our knowledge of coronal heating and of coronal conditions as well as
processes affecting solar-wind acceleration, by providing information
on:
(a) Electron temperature and temperature gradient in the region of the
corona (including magnetically closed regions in which mass-ejection
coronal transients appear to originate) where the observed solar wind
is accelerated. This information is obtained by comparison of the
measured distribution of ionization states of C, O, Mg, Si, and Fe.
(b) Physical processes in the acceleration region (e.g. ion heating by
wave-particle interactions, acceleration by frictional coupling)
obtained from measurements of the composition, temperature, and mean
speeds of heavy solar wind ions.
(c) Locations of the sources of the solar wind provided by composition
and charge-state measurements of solar wind heavy ions at different
latitudes.
(d) Compositional variations in the solar atmosphere provided by
composition measurements, including for the first time elements like
carbon, magnesium and calcium, over a wide range of solar latitudes
and for all solar wind flow conditions.
(e) Average solar abundances obtained from comprehensive composition
and charge-state measurements of solar wind ions.
2.2. PLASMA AND ACCELERATION PROCESSES IN THE SOLAR WIND.
Plasma processes in the solar wind are expected to vary with latitude.
At low latitudes, where effects of solar rotation are important,
stream-stream interactions and the spiral nature of the heliospheric
magnetic field have a controlling influence on the plasma processes.
In contrast, at high latitudes stream-stream interactions should be
much weaker, and the fields radial. It may also be that the more rapid
decrease with distance of the radial magnetic field over the solar
poles results in a higher [beta] plasma (ratio of thermal to magnetic
pressure) than near the ecliptic plane.
Our measurements of the kinetic temperatures of solar-wind ions will
provide an important probe of the interplanetary plasma processes that
occur as a function of latitude. The response of the kinetic
temperatures of solar-wind ions with different charge/mass ratios can
be used to study the dominant instabilities which are being excited
during stream-stream interactions, or by heat-flux instabilities, or
by other processes.
Detailed studies of acceleration processes in the solar wind which
lead to the ~MeV/nucleon particles observed in interplanetary space
will be possible with the SWICS. Measurements of suprathermal tails of
proton, He^++, C^6+, O^6+, and Fe^7+ to Fe^9+ energy spectra, and
comparison of compositions and charge-state distributions of solar
wind and accelerated ions will reveal whether these ions are
accelerated directly out of the local solar wind or have a different
source of origin, and will indicate how the acceleration and
propagation mechanisms depend on particle rigidity. Stream-stream
interactions, which are the most likely candidates for the
acceleration of these MeV ions, should vary markedly with latitude
making these studies especially interesting and unique with Ulysses.
2.3. INTERSTELLAR IONS.
Interstellar neutral gas, which probably consists mainly of H, He, N,
O, and Ne, is swept into the heliosphere by the motion of the Sun
through the interstellar medium. In the heliosphere these interstellar
atoms are singly ionized by photo-ionization from solar UV and by
charge exchange with the solar wind, and are then convected outward by
the solar wind as they gyrate about the heliospheric magnetic field.
The speed of these pick-up ions in the spacecraft frame should range
between approximately zero and twice the solar-wind speed.
On Ulysses, interstellar He will be available for study throughout the
mission, since neutral interstellar He can penetrate to within 0.5 AU.
Other interstellar neutrals such as O and Ne will partially penetrate
to within a few AU of the Sun, and thus should also be seen with the
SWICS near 4 to 5 AU.
The initial distribution of the interstellar ions, immediately
following their ionization, tends to be quite unstable. How fast and
by what mechanism these ions are accommodated into the main solar wind
gas remains largely unexplored and will be studied using measurements
provided by SWICS which can readily identify these pick-up ions and
measure their energy spectra.
2.4. JOVIAN STUDIES.
We expect to obtain important new information concerning the
composition of the thermal and suprathermal plasma in the Jovian
magnetosphere and to make the first direct ionization state
measurements. This information addresses questions of the relative
strengths of the various plasma sources, and plasma heating,
acceleration, and transport processes within Jupiter's magnetosphere.
SWICS may also shed light on the question of the origin (solar wind or
Jovian) of suprathermal and energetic ions observed by Voyager 1 and 2
upstream of the Jovian bowshock. Because of the nature of the
encounter trajectory, the SWICS scan will include the corotation
direction outbound but not inbound. Therefore, inbound only the hot
plasma and suprathermal components (kT[hot] ~ 30 keV from the LECP
experiment on Voyager [Krimigis et al. 1979]) will be sampled while
outbound the cold corotating ions will be observed as well.
The SWICS energy per charge range of 0.16-60 keV/e overlaps that of
the Voyager instrumentation and fills the keV/e to 30 keV gap which
existed between the Voyager Plasma Science (PLS) and Low Energy
Charged Particle (LECP) experiments. It will provide the first and for
a long time the only detailed charge state and species information for
the hot plasma and suprathermal components.
Among the specific questions which will be addressed by SWICS is the
makeup of the ions that originate from Io, one of the strongest Jovian
plasma sources identified by the presence of sulfur, oxygen, and
sodium ions. By determining mass as well as mass per charge, SWICS can
help resolve ambiguities inherent in the Voyager measurements which
could not distinguish between e.g. O^+, and S^++, and between O^+, and
O^++. Although SWICS is not expected to make high resolution
measurements in the inner magnetosphere because of background
considerations, its measurements of Iogenic ions in the outer
magnetosphere will shed light on relative charge state and chemical
abundances. SWICS will also search for molecular ion products arising
both from SO[2] and H[2]0. Products of H[2]0 would be evidence of
plasma originating from the icy Jovian satellites. A determination of
the abundance of H[2]^+ and H[3]^+ is important in evaluating the
relative source strength of the ionosphere compared to Io and the
solar wind. The discovery of these molecular ions by Voyager at
energies above 0.5 MeV/nucleon (Hamilton et al., 1980), established
the Jovian ionosphere as an important plasma source.
Energetic ion increases in the tens of keV range, observed upstream of
the bowshock at Earth and by Voyager 1 and 2 at Jupiter (e.g. Ipavich
et al. 1981, Zwickl et al. 1980), (e.g. Lee 1982) are believed to be
either solar wind ions accelerated at the bow shock by the first-order
Fermi mechanism, or particles leaking from the magnetosphere with
little additional acceleration (e.g., Krimigis et al. 1985). The
resolution of this controversy will come from high resolution
composition and charge state measurements made by SWICS. Heavy ions
(e.g. O, S, and Fe) will be distinguished from protons, and high-
charge state heavy ions from the solar wind can be distinguished from
the low charge state heavy ions of magnetospheric origin. Voyager
composition measurements started at much too high an energy (~ 200
keV/nucleon) to make the definitive composition measurements and
Galileo measurements will not give charge state information.
3. Instrument description.
3.1. PRINCIPLE OF OPERATION.
The SWICS sensor is based on the technique of particle identification
using a combination of electrostatic deflection, post-acceleration,
and a time-of-flight (TOF) and energy measurement (Gloeckler 1977)
(Gloeckler & Hsieh 1979). Figure 1 shows schematically the operating
principle of the sensor and the functions of the five basic sensor
elements employed:
* Ions of kinetic energy E, mass m and charge (ionization state) q
enter the sensor through a large area, multi-slit collimator
which selects proper entrance trajectories for the particles.
* The electrostatic deflection analyzer serves as an energy-per-
charge (E/q) filter, allowing only ions within a given energy-
per-charge interval (determinded by a stepped deflection voltage)
to enter the TOF vs Energy system.
* Ions are post-accelerated by a ~ 30 kV potential drop just before
entering the TOF vs Energy system. The energy they gain is
sufficient to be measured adequately by the solid-state
detectors, which typically have a ~ 30 keV energy threshold. An
energy measurement is essential for determining the elemental
composition of an ion population and ions with energies below ~
30 keV must be accelerated if their mass is to be identified.
* In the time-of-flight system the velocity of each ion is
determined by measuring the travel time [tau] of the particle
between the start and stop detectors separated by a distance of
10 cm.
* The particle identification is completed by measuring the
residual energy of the ions in a conventional low-noise solid-
state detector.
From simultaneous measurements of the time-of-flight [tau] and
residual energy E[meas], and a knowledge of the deflection system
voltage and hence the E/q, and of the post-acceleration voltage V[a],
we can determine the mass (m), charge state (q) and incident energy
(E) of each ion as follows:
m = 2([tau]/d)2 (E[meas]/[alpha])
m/q = 2([tau]/d)2 (V[a] + E'/q) ~= (2([tau]/d)^2)V[a]
q = (E[meas]/[alpha])/(V[a] + E'/q)
~= (E[meas]/[alpha])/V[a]
E = q.(E/q)
where d is the flight path. E'/q takes account of the small energy
loss of ions in the thin foil of the start-time detector (Ipavich et
al. 1982) and [alpha] (❮1) is the nuclear defect in solid-state
detectors (Ipavich et al. 1978). The approximate expressions for q and
m/q hold for typical solar wind ions.
3.2. THE INSTRUMENTATION.
The SWICS experiment consists of three separately mounted units which
are electronically interconnected as shown schematically in Figure 2.
The three units are the Sensor, designated GLG-2A, the Post
Acceleration Power Supply (PAPS), called GLG-2B, and the Data-
Processing Unit (DPU), labelled GLG-1. These units, indicated in
Figure 2 by solid boxes, in turn contain various subsystems which will
be described more fully below. The subsystems and the institutes
responsible for their design, fabrication and testing are also shown
in Figure 2.
3.2.1. Sensor.
A simplified cross-section of the SWICS sensor GLG-2A, consisting of
the deflection analyzer and the high-voltage bubble, is shown in
Figure 3. The cylindrical-shaped high-voltage bubble, to which a post-
acceleration voltage of up to 30 kV may be applied, contains the TOF
telescope and a proton-alpha particle detector, the analog electronics
and the sensor power supplies. Each of these subsystems is supported
by a G-11 insulator bulkhead and enclosed by a machined-aluminium
container with its outer, parylene-coated surface separated from the
parylene-coated inner surface of the outer housing by a 6 mm vacuum
gap. The ultra-clean TOF compartment is physically isolated from the
bubble electronics, with venting provided through the entrance slits
and the collimator-deflection system. Digital signals are transferred
to the DPU across the 6 mm gap by six opto- couplers. The opto-coupler
openings in the housings also serve as venting ports for the sensor
electronics and power supplies. Power is supplied to the high-voltage
bubble by means of an isolation transformer through a six-pin, high-
voltage feedthrough (not shown) connected to the upper compartment.
The photograph of the sensor (Fig. 4) shows the outer configuration of
the cylindrical bubble housing, the opto-coupler box, and the
deflection system with collimator opening covered by a dust/acoustic
protective cover which swings open after launch. The gold-plated,
cylindrically-shaped container houses the -30 kV supply. The sensor is
mounted on the earth-oriented side of the spacecraft platform, in the
same orientation as shown in the photograph.
Deflection analyzer. The three-dimensional configuration of the
deflection analyzer may be visualized by revolving the cross-sectional
view shown in Figure 3 by 69 [deg.] about the symmetry axis of the co-
axial cylindrical containers. A single conical collimator services the
two separate deflection regions of the analyzer. The multi-slit
collimator is similar in construction to collimators on our ISEE
instruments (Hovestadt et al. 1978) and the AMPTE instrument
(Gloeckler et al. 1985), and allows us to extend the upper energy
limit of our analyzer system while maintaining a reasonably large
geometrical factor (~ 10^-2 cm^2). The widths of the individual
channels in the collimator are such as to limit dispersions in the
analyzer and flight-path differences in the TOF system to ❮ 0.5%. The
two inner deflection plates are connected to separate outputs of a
variable voltage supply housed immediately below the deflection system
(Fig. 3) which increments the deflection voltage of both plates
simultaneously in logarithmic steps. The maximum voltages on the upper
and lower deflection plates are + 1 kV and + 6 kV, respectively.
Serration, black-coating and light traps are used to eliminate
reflection of visible and UV radiation into the TOF system. Figure 5
shows a top view of the sensor with the electrostatic deflection
plates and collimator plates exposed.
The smaller (upper) of the two deflection analyzer regions
(proton/helium channel) normally covers an energy range from 0.16 to
14 keV/charge, has a resolution of ~ 4% and will routinely analyze
solar-wind protons, He and heavier ions. These ions will be post-
accelerated and will be counted by a single rectangular solid-state
detector at two threshold levels (20-45 keV, and above 45 keV)
corresponding to the energies of post-accelerated solar-wind protons
(~ 30 keV) and post accelerated He and heavier ions ( ❯=60 keV). As
the voltage is stepped over the full range, this system provides
separate E/q spectra for solar-wind protons, and for solar-wind He
plus heavier elements, allowing us to determine in a simple manner,
and at all solar-wind temperatures, the bulk speed, density and
temperature of H^+ and He^++ in the solar wind.
The larger (lower) deflection analyzer (main channel) has a ~ 5%
energy/charge resolution and is used for the full m vs m/q analysis of
solar-wind He and heavier ions and of suprathermal ions, in the range
0.65-60 keV/charge. The TOF vs E system is placed behind the exit slit
of the analyzer and inside the high-voltage bubble (Fig. 3). At any
given voltage step, the analyzer passes ions that have equal (to
within the 5%) energy per charge. These ions are then post-accelerated
and their mass, charge state and energy measured by the TOF vs E
system as described below.
Time-of-flight vs energy system. An important advantage of the TOF
technique which measures the velocities of ions over a system which
selects a narrow range of velocities (using, for example, crossed
electric and magnetic fields) is that stepping over a velocity range
is not required in our instruments. The TOF system accepts a wide
range of velocities (or m/q ratios) simultaneously, resulting in
factors of 10 to 20 increases in both the time resolution and
sensitivity. A second advantage is that coincidence measurements used
in TOF systems reduce background levels many orders of magnitude below
the typical 10^-2 to 1 count/s background measured in, for example,
the singles rates of solid-state detectors on a typical spacecraft.
Figure 6 shows the cross-section of the SWICS TOF vs E assembly
consisting of a "start" and "stop" detector 10 cm apart. The start and
stop signals are derived from secondary electrons (Gloeckler & Hsieh
1979) that are released with a mean energy of a few eV when an ion
enters or leaves a solid surface. The surface material used for the
start detector is a thin foil (~ 3 [micro]g/cm^2 carbon foil supported
on a 90% transmission nickel grid), and for the stop detector the gold
front surface of the Au-Si solid-state detector. The secondary
electrons from the start and stop detectors are accelerated to ~ 1 kV
and then deflected by a system of acceleration gaps and deflection
surfaces as shown in Figure 6 and strike the respective Micro Channel
Plate assembly, each of which consists of a rectangular, curved-
channel MCP. A common supply (1 kV) is used to both accelerate and
deflect the electrons. The output signals from the start and stop MCP
assemblies are impedance matched and capacitively coupled across ~ 4
kV in order to keep the foil and solid-state detector at local ground
potential. The MCPs are normally biased at ~ 3 kV to operate at a gain
of 2 x 10^6. This bias voltage (and hence the gain) is adjustable by
ground command.
The top view of the TOF vs energy telescope (Fig. 5) shows the
positions of the start and stop MCPs, the three solid-state detectors,
and the curved carbon foil. The wide field-of-view and the three
detectors are necessary to provide look directions towards the Sun
over the entire 0 [deg.] to 60 [deg.] range of Sun-spacecraft-Earth
angles. The slight difference in the flight path of the secondary
electrons introduces a timing uncertainty of ❮0.2 ns FWHM, which is
smaller than the ❮0.5 ns FWHM resolution of the analog electronics.
Ions are practically unaffected by the electric fields of the TOF
assembly, because of their higher energy. The path length dispersion
[DELTA]d/d for ions is negligible in the plane of Figure 6 and ❮0.005
FWHM in the other direction (Fig. 5).
The residual energy is measured by one of the three rectangular solid-
state detectors. Table 1 gives additional details for the SWICS TOF vs
E assembly. The TOF vs E telescope is shown in Figure 7.
3.2.2. Electronics.
The SWICS electronics consists of an analog subsystem which is built
into the central compartment of the high-voltage bubble (Fig. 3), the
DPU and the low-voltage power supply, both housed in the separately
mounted GLG-1 box, a variable deflection voltage supply and a
detector/MCP bias supply both contained in the sensor (Fig. 3), and
the -30 kV post-acceleration supply in its own cylindrical housing.
These subsystems are shown schematically in Figure 2.
Analog electronics. A simplified block diagram of the SWICS analog
electronics is shown in Figure 8. The main function of these
electronics is to measure the time-of-flight [tau] and energy E of
ions triggering the TOF vs E system. In addition, solar-wind protons
and He are quickly identified and counted, and a number of coincidence
conditions are established and their occurence counted.
Time-of-flight measurement. Each MCP assembly output is capacitively
coupled to a fast preamplifier whose function is to accept the 0.9 ns
rise-time MCP output signals, shape, amplify, and feed them into a
fast timing discriminator using tunnel diodes. The output signals from
the start and stop timing discriminator are used as inputs to the
Time-to-Amplitude Converter (TAC) which will produce: (a) an output
pulse whose amplitude is proportional to the time interval between the
trigger of the start and stop discriminators (T signal), and (b) a
logic pulse (valid [tau]) provided the stop signal follows the start
signal in ❮200 ns The T signal is stretched and pulse-height-analyzed
by a 10-bit "Time" Amplitude-to-Digital Converter (T-ADC) with a 40
[micro]s conversion time. The valid [tau] logic pulse is used to
establish logic conditions and increment counting rates. We have
measured the overall timing resolution of the analog electronics to be
❮0.2 ns when both the start and stop MCP pulses exceed 100 mV.
Energy measurement. The output of each of the three solid-state
detectors of the TOF telescope is coupled to a low-noise (5 keV FWHM)
preamplifier and shaping amplifier (unipolar pulses of 1 [micro]s
duration at 10%). All amplifiers have been hybridized to minimize
weight and reduce cross-talk. The output signals of each amplifier
chain (E-signals) is pulse-height-analyzed by a common 8-bit "Energy"
Amplitude-to-Digital Converter (E-ADC) and are fed to respective
threshold discriminators, whose output is used to identify the
triggered detector and to establish logic conditions and increment
counting rates.
Solar-wind proton/helium channel. The output of the low-noise H/He
solid-state detector also goes through a preamplifier/shaping-
amplifier chain which then feeds two threshold discriminators (20 keV
and 45 keV) whose outputs increment the "proton" and "He" rate and
channels, respectively. It is possible to command the instrument into
a mode such that the output of the H/He detector is analyzed instead
of that of the three main telescope detectors.
Basic rates, logic conditions, inflight calibrator. Table 2 lists the
data items (rates and pulse-heights) generated by the analog
electronics. Pulse-height-analysis of the T signal is normally started
by a valid signal whether or not an energy signal is present. Pulse-
height-analysis of the E signal normally requires a triple coincidence
condition (start-stop and energy). It is possible, however, to change
the T-ADC analysis logic by ground command to require also a triple
coincidence for its analysis. An inflight calibration provides either
on command, or at pre-programmed time intervals, a sequence of timing
and amplitude pulses which are fed, respectively, to the fast
amplifiers or preamplifiers of each MCP or solid-state detector. When
the instrument is being calibrated the trigger logic prevents all
except the calibrate pulses from being analyzed.
Data-processing unit. The basic data (Time and Energy pulse heights)
provided by the SWICS sensor lend themselves to straightforward on-
board processing which simplifies ground-based data reduction. The DPU
performs this function and provides the interfaces to the SWICS sensor
and analog electronics and to the spacecraft (Fig. 2). The other
principal functions of the DPU are to: (a) execute fast classification
of ions according to the ion mass (m) and mass per charge (m/q); (b)
collect and store count-rate and pulse-height data, determine event
priority, and execute appropriate event sequencing, compress the
contents of each counting-rate register into an 8-bit floating-point
representation, format all data into 8-bit words, and transfer this
information to the spacecraft; and (c) perform all necessary control
functions for the experiment, accept and execute ground commands,
monitor the experiment status, and execute an on-board calibration
sequence.
Fast on-board data processing (m vs m/q classification). For every ion
for which E and T pulse heights are available, the m and m/q are
calculated using fast (~ 50 [micro]s) look-up-table techniques which
establish a correspondence between the measured T and E pulse heights
and the stored positions of the m and m/q surfaces in the T vs E
parameter space. In order to reduce drastically (by a factor of ~ 100)
the excessive storage capacity requirements inherent in a "brute-
force" look-up technique (required ROM capacity of ~ 22 Mbits), we use
a first-order Taylor expansion for each coarse grid of the T vs E
plane, as illustrated in Figure 9 for the mass classification.
The mass m(T, E) of an ion is computed from the T and E pulse heights
produced by this ion using
m(E, T) = m[0](E[0], T[0]) + dm/dE(E[0], T[0])*[DELTA]E +
dm/dE(E[0], T[0])*[DELTA]T
where E[0], T[0] are the most significant bits of the E and T pulse
heights, [DELTA]T and [DELTA]E are least significant bits, and
m[0](E[0],T[0]), dm/dE(E[0],T[0]) and dm/dT(E[0],T[0]) are appropriate
elements stored in a ROM look-up table.
Matrix rates and matrix elements. The m and m/q values returned by the
fast classifier are used to increment appropriate storage elements (MR
registers) corresponding to counting rates of selected ion species
(Tab. 2), and of the 512 matrix elements contained within the boxed
regions of the mass vs mass/charge plane shown in Figure 10.
Direct Pulse-Height Analysis data and priority selection. The most
detailed information about the composition, arrival directions and
energy of ions is contained in the 24-bit PHA words (eight bits for
energy, ten bits for TOF, three bits for one of eight sectors, two
bits for one of three detectors, and one bit for priority).
The DPU collects and formats the required information for each PHA
word, establishes priority categories, and compiles a string of PHA
words arranged sequentially such that a balanced ratio of high and low
priority events is always telemetered. Three priority categories are
defined as shown in Figure 10. Category-I (Range-0) are elements
classified to have a mass less than 8.7, category-II (Range-1) are
those with mass greater than 8.7, and category-III (Range-2) are low-
charge-state heavy elements (m/q ❯ 3.3), likely to be interstellar
O^+, Ne^+, etc, which do not trigger the solid-state detectors and
hence produce only a TOF pulse height. Such events are labelled "mass-
zero" events because only their mass/charge can be measured. Normally,
category-I events are assigned lowest priority and category-III events
highest priority. Other priority schemes, including a rotating
priority, can be implemented by ground command.
Housekeeping data, analog performance parameters, rate data (Tab. 2),
eight of the 512 matrix elements and a maximum of 30 (eight at reduced
bit rate) PHA events are telemetered once every 12 s (one spin
period). Priority selection of the 30 (or 8) telemetered PHA events is
accomplished in three steps: (a) the first 30 (or 8) events are
accumulated regardless of priority, (b) the next 30 (or 8) events are
restricted to middle- and high-priority events and replace the first
group of 30 on a one-to-one basis, and (c) the final 30 (or 8) events
are restricted to the highest priority, and again replace the second
group of 30 on a one-to-one basis. Depending on the event rate, this
process may be terminated at any, and within any, of the three steps
at the end of the 12 s period between readouts It should be noted that
priority selection affects only the direct pulse-height data.
3.2.3. Instrument requirements.
Mass and power. The mass of the complete SWICS instrument, including
interconnected harness and thermal blankets for GLG-2A and B (sensor
and high-voltage supply) is 5584 g, and the total raw power
requirement (except for heater power) is 3950 mW (Table 3).
Telemetry and command. On-board processing reduces considerably the
telemetry that would otherwise be required to send back the
information acquired by the SWICS sensor. The telemetry required to
transmit the basic rates, matrix rates, m vs m/q elements and direct
pulse heights is given in Table 2. The total bit rate is 88 bit/s in
the "tracking" mode and 44 bit/s in the storage mode. In addition,
five analog performance parameters are read out. The instrument
requires three on-off commands (instrument power, post-acceleration
supply power, and heater power), three 16-bit command words, and a
redundant pair of commands to initiate opening of the acoustic cover.
Cleanliness and thermal. Because of the susceptibility of the MCPs and
solid-state detectors to contaminants and because of use of thin foils
in this experiment a dust/acoustic cover is provided for the sensor.
This cover sealed the collimator opening and was commanded to swing
open after the spacecraft was injected into its orbit. To avoid damage
to the MCPs prior to launch, the instrument was purged continuously
with dry nitrogen through ports provided in the sensor.
Thermal-design requirements for the GLG-2A sensor are an orbital
temperature of 0 [deg.]C to -25 [deg.]C, preferably -10 [deg.]C to -20
[deg.]C. The thermal requirements are driven by solid-state-detector
operating and survival temperature limits, and are achieved through
use of thermal reflecting coatings, blankets and radiators, and two
heaters.
3.2.4. Instrument capabilities and characteristics.
The SWICS instrument is capable of measuring the solar-wind elemental
and charge-state composition under all conceivable solar-wind
conditions. The sensitivity and dynamic range of the instrument are
such that the mean speeds, temperatures and densitites of all major
elements in the solar wind may be determined with a time resolution of
13 min to a few hours.
Energy range. The combined energy range of the main channel and the
H/He channel extends from 100 eV/e (140 km/s protons) to 60 keV/e
(3400 km/s protons; 1285 km/s Fe^+8). This large dynamic range of 600
in energy per charge will allow us to carry out solar-wind composition
measurements under all possible flow conditions, as well as to study
the suprathermal tails of the distribution functions of for example H,
He and O. Table 4 lists the energy ranges and resolution (3.65 or
7.44% selectable by command) for the four modes of operation of the
SWICS instrument.
Sensitivity. The multi-slit collimator used in the SWICS makes it
possible to obtain a relatively large geometrical factor (~ 10^-2
cm^2) and a wide field-of-view without sacrificing the energy
resolution of the deflection analyzer. The counting efficiency in the
TOF system depends on the degree of scattering of ions in the front
foil (most pronounced for low-energy heavy ions) and number of
secondary electrons produced by ions at the surface of the foil and
solid-state detector (lowest number for higher energy protons). Pre-
flight calibrations indicate that the counting efficiency for triple-
coincidence analysis is typically in the range 30 to 80% for ions
heavier than He in the energy range of the SWICS (depending on the MCP
bias level). The efficiencies for counting protons and He are lower;
however, the solid state detector in the H/He channel will identify
and count H, He and heavier ions with nearly 100% efficiency (at 30 kV
post-acceleration).
Intensity dynamic range. With the SWICS instrument we are able to
achieve an intensity dynamic range of ~ 10^9 because: (a) the most
intense solar-wind fluxes (protons) are generally measured only in the
H/He channel; (b) the rare elements are analyzed in the TOF vs E
system with high priority; and (c) the high immunity of background
makes it possible to detect and identify rare elements and ions. The
largest contributors to background are RTG [gamma]'s and neutrons, and
penetrating (❯=10 MeV) solar-flare protons. For example, in a typical
solar-flare particle event the flux of ❯10 MeV protons is ~ 20 cm^-2
s^-1 sr^-1, which will result in a background counting rate in a
single detector in excess of 100 count/s. The triple coincidence
technique and multi-parameter analysis used in the SWICS reduces the
instrumental background due to this source to ~ 10^-4 count/s in each
of the time vs energy matrix elements. In testing a prototype TOF
telescope exposed to [gamma]-ray fluxes of an RTG simulator we found
triple-coincidence rates of ~ 10^-4 count/s. Using integration times
of several hours and being able to further correct the detailed PHA
data for any residual background, we should be able to measure minimum
solar-wind fluxes of between ~1 and 20 cm^-2 s^-1 depending on
spacecraft orientation. The maximum proton flux of ~ 10^9 cm^-2 s^-1
we can measure is determined by the peak counting rate of our H/He
detector (5 x 10^5 count/s, the proton temperature, and the energy
bandwidth and geometrical factor of the deflection analyzer.
Mass and mass per charge resolution. We have determined the mass and
mass/charge resolution of the SWICS for every major ion species using
all known aspects of the instrument. These include effects due to: (a)
deflection analyzer system resolution and dispersion; (b) electronic
noise in the TOF measurement and path-length dispersion of secondary
electrons; (c) path-length differences of ions in the TOF system; (d)
energy dispersion associated with nuclear defects in solid-state
detectors and electronic noise in the energy measurement; and (e)
timing dispersions caused by energy straggling of ions in the carbon
foil. These effects which have been measured or are determined by the
geometry of the system are combined in quadrature to give the FWHM
resolutions in mass, [DELTA](m) and mass/charge, [DELTA](m/q). Table 5
summarizes the mass and mass/charge resolution in the SWICS for common
solar-wind ions (at 30 kV post-acceleration).
Modes of operation. Voltage cycle mode. The energy-per-charge dynamic
range and step resolution of the instrument are commandable by six
predefined voltage cycle modes that are available for the
electrostatic deflection analyzer system. A deflection voltage cycle
consists of 64 voltage steps, resulting in an instrument duty cycle of
approximately 12.8 minutes at spacecraft bit rates of 1024 and 512
bps, and at the nominal spin rate of 12 seconds. (The deflection
analyzer voltage steps once per spacecraft spin at these telemetry bit
rates). The deflection voltage plates of the Main and H/He channels
step in concert, with the H/He channel offset by a factor of four
lower in energy per charge from the Main channel. The deflection
voltage is automatically stepped down (or up, after step reversal)
according to the mode selected by command. Two of the commandable
voltage modes cover primarily the suprathermal energy regime (with
Main channel energy-per-charge ranges of 59.6-6.3 keV/e and 40.4-4.3
keV/e, respectively). The four operational modes most likely to be
used in flight, covering the solar wind energy-per-charge regime, are
given in Table 4. For each mode, the corresponding energy-per-charge
and velocity ranges for different ion species are given for the Main
and H/He channels. Depending upon the mode, the voltage step spacing
is either 7.44% or 3.65%, thus providing a choice between finer
resolution in the energy-per-charge steps or a larger dynamic range.
Automatic step reversal mode. In order to extend the life of the
microchannel plates used in the Main channel, the total fluence on the
plates is restricted through the use of a commandable "automatic step
reversal" in the deflection voltage cycle. This features takes
advantage of the fact that the solar wind velocity is roughly the same
for all ion species, hence the energy-per-charge spectrum is well-
ordered in mass per charge. The SWICS starts a voltage cycle at the
highest voltage step (and hence highest energy per charge) of a given
voltage cycle mode. As the deflection system steps down in energy per
charge, the minor ions and helium ions are observed before the system
steps into the energy-per-charge domain of the high fluence solar wind
protons. A maximum allowable counting rate for the start signal
microchannel plate is set by ground command, at any one of eight
selectable rate-limit values (ranging from 256 to 524, 288 counts per
spin). If during one spin the start signal rate (the FSR) exceeds the
selected value, the step direction of the deflection voltage cycle is
reversed. The deflection voltage is then stepped up until a full cycle
of 64 steps is completed (if the highest voltage step is reached
before the end of the cycle, the deflection voltage remains in that
step until the cycle is completed). An example of the instrument
undergoing step reversal under flight conditions is given in Figure 12
(see Sect. 5).
The automatic step reversal assures a conservative level for the total
fluence on the microchannel plates. Except under unusual conditions
(solar-wind densities in excess of 25 cm^-3) the typically selected
rate limit will not be exceeded until the TOF vs E system completely
analyzes He and begins measuring into the proton peak. Because the
H/He channel is offset in energy-per-charge from the Main channel by a
factor of four, the H/He channel normally steps through the protons as
well as helium and the heavier ions before step reversal is initiated
by the fluence level measured in the Main channel. The rate-limit
condition is overridden automatically once every 64 voltage cycles
(about every 14 hours for nominal spin periods and telemetry bit rates
of 1024 or 512 bps) to allow a complete spectrum. The Automatic Step
Reversal mode can be disabled by ground command.
Post acceleration power supply mode. Ions with the correct energy per
charge to get through the deflection analyzer system then pass through
a commandable potential drop before entering the TOF vs. E system of
the instrument. This post acceleration after electrostatic analysis
raises the energy of the ion by an amount V[a]*q, which is typically
sufficient to trigger the solid state detector and allow energy
measurements. There are sixteen commandable post acceleration voltage
levels, ranging from 8 kV to 30 kV. The Data Processing Unit can
correctly classify ions with post acceleration voltages that are ❯=15
kV. It can also classify events for V[a] = 0 (i.e., post-acceleration
off) for deflection voltage steps ❯= 100 (E/q ❯= 15.5 keV/e in the
Main channel). This last feature allows a partial recovery of the
science data in the event of a failure of the post acceleration power
supply. During the early part of the mission (November to December,
1990), the post acceleration level was gradually stepped up to its
current level of 22.9 kV. Although this voltage level is insufficient
to obtain triple coincidence events on solar wind protons and some
fraction of solar wind alphas, it does permit triple coincident
information on most minor ions.
H/He channel pulse height mode. In the event of a failure of the Main
channel's TOF system, there is a partial recovery mode in the
experiment that replaces in the telemetry stream the direct pulse
height data from the Main channel with direct pulse height data taken
from the H/He channel. (Normally, the H/He pulse height data are not
transmitted in the telemetry). In this operational mode, one has the
standard electrostatic deflection (E/q) and total energy (E)
instrument, combined with post acceleration .
4. Instrument calibration.
Thorough calibration of the instrument was critical because the SWICS
is a complex detector system which combines three distinct subsystems:
(a) the collimator/deflection system (providing E/q); (b) the time-of-
flight sensor (providing particle velocity); and (c) solid-state
detectors (providing total energy). Data from these subsystems must be
reliably combined (both by the on board DPU and on the ground) to
infer the incident energy, mass, and mass per charge of incoming ions.
Calibrations of the SWICS engineering-model (EM), flight spare (FS),
flight unit (FU), and refurbished flight unit sensors were performed
before flight.
At the time SWICS was being tested no single calibration facility was
capable of simulating the solar-wind ions that the sensor is now
detecting in space. This would have simultaneously required: low
incident E/q (0.1-60 keV/e); high charge states (q ~ 1 to 20); and
elements from H through Fe. It was therefore necessary to use several
complementary facilities to provide our overall instrument
calibration. Particle beam calibrations of the instrument and
subsystems were performed using the DC accelerator/mass spectrometer
at the University of Bern, the DC accelerator at the MPAe/Lindau, and
the Van de Graaff accelerator at Goddard Space Flight Center.
The accelerator at Bern was the only facility at which we calibrated
the fully assembled SWICS. Here we measured the efficiencies, geometry
factors, energy and angular response of the collimator/deflection
system over the full energy per charge range of the sensor. The
accelerator facility was ideally suited for SWICS in that it covered
an energy range of a few eV to 60 keV/e and provided a large-area,
highly parallel beam of ions (Ghielmetti et al. 1983). Efficiencies
were determined for the microchannel plates for a variety of ion
species and for different bias levels of the MCPs. The ion species
tested included H^+, H[2]^+, He^+, He^+2, O^+, C^+, N^+, Ne^+3, Ar^+4,
Kr^+4. Relative efficiency data for the flight configuration
microchannel plates are shown in Figure 11 for Neon.
5. Post-launch performance.
The SWICS instrument reached its normal mode of operation in December
9, 1990 following a turn-on sequence that included a gradual increase
in the post-acceleration voltage to the present value of 22.9 kV. The
experiment is operating in space as designed and is returning data of
excellent quality unavailable in previous explorations of the solar
wind. In particular, background suppression techniques used to
minimize drastically if not eliminate contributions from UV, RTG, etc.
have proven to be extremely effective, and the SWICS's capability to
resolve all major heavy elements in the solar wind and measure their
charge states has now been demonstrated. Below we will provide just a
few examples of essentially flight raw data processed on-board by the
DPU.
The four panels of Figure 12 show the energy per charge vs time color
spectrograms for the H/He channel (top), He^++, O^6+, and Si (bottom)
respectively. Vertical white bars indicate data gaps, blank areas
represent zero counts and the color bar on the right codes the
logarithm of counts in a given energy per charge bin. Displays of this
type will be produced routinely and are useful for an overview of
solar wind flow conditions throughout the mission. In the top panel
each ~ 13 minute, 64-step scan from high to low energy per charge
shows resolved peaks due to solar wind H^+ and He^++ measured with the
solid state detector of the H/He channel. Variations in the bulk
speed, density, and kinetic temperature are easily discerned in the
data shown in all four panels. Effects due to step reversal are seen
in all but especially the three bottom panels. Step reversal occurs
when the count rate in the start detector of the time-of-flight system
exceeds a value selectable by command (see Sect. 3.2.4). At that time
the stepping sequence is reversed; that is, the deflection voltage is
sequentially increased until the 64-step cycle is completed. Since
step reversal most often occurs before the peak of the proton
intensity is reached, some of the low-energy portion of the energy per
charge spectrum is retraced for alpha particles and heavier ions in
the main (time-of-flight) system. The fraction of the spectrum
retraced depends on the mass/charge of the ion and the bulk speed,
density and kinetic temperature of protons (see Fig. 12, three bottom
panels). Because the proton-alpha channel covers an energy/charge
range four times lower than the main channel, protons are measured by
that system before (and sometimes after) step reversal, as can be seen
in the top panel of Figure 12. The step reversal mode is implemented
in order to limit the total counts accumulated in the star
microchannel plate durign the five year period of the Ulysses mission.
In Figure 13 we display the differential intensity spectra of H^+ and
He^++ respectively, time-averaged over a two day period. The
differential fluxes for each ion species are derived from the
corresponding matrix rates generated from the data classified by the
DPU. The smooth curves represent convected maxwellian distributions
each having the same bulk speed of 410 km/s and a temperature/mass,
T/m, of 13.5x10^4K. This single maxwellian is a good fit to the main
portion of the measured spectra for each species indicating that these
ions have a common bulk speed and kinetic temperature per mass.
Notice, however, that the differential spectra of protons and alpha
particles have pronounced, non-maxwellian, high-energy tails.
The highest-resolution mass and mass/charge information is derived
from the direct pulse-height data. These data are used by the DPU for
on-board mass classification and the generation of matrix rates and
matrix elements, and a sample of a most 30 direct pulse-height events
is transmitted every spin. Thus, fairly long-term averages are
required to accumulate enough statistics to reveal the presence of the
less abundant heavy ion species. An example of a mass vs mass per
charge matrix accumulated over a nine day time period and summed over
all voltage steps is shown in Figure 14. Mass and mass/charge values
were computed from each pair of energy and time-of- flight pulse-
heights using algorithms identical to those employed in the DPU. The
color scale was adjusted to reveal the presence of rarer ions. In
addition to the more abundant ions C^6+, C^5+, O^7+ and O^6+, the
presence Ne^8+, Mg^10+, C^4+, charge states 7, 8 and 9 of Si, and
charge states 7, 8, 9, 10 and 11 of Fe are also visible.
The mass and mass/charge resolution capability of SWICS is illustrated
in Figure 15 using matrix element [Fig. 15(a)] and direct pulse-height
data [Fig. 15(b), (c), and (d)] summed over all voltage steps during a
two day period. In Figure 15(a) the mass distribution for ions with
mass/charge values between 1.76 and 2.11 amu/e demonstrates that C^6+
is clearly resolved from He^++. Figure 15(b) shows the mass
distribution in the mass/charge range of 2.45 to 2.55 amu/e,
indicating that C^5+ (m/q = 2.4) and Ne^8+ (m/q = 2.5) are resolved.
In Figure 15(c) we show the mass per charge distribution in the mass 8
to 10 amu range. In addition to the three charge states of carbon that
are well resolved, some fraction of O^6+ spills over into the 8 to 10
mass range (see also Fig. 14). Despite this spillover, the excellent
mass/charge resolution allows us to easily separate oxygen from
carbon. Finally, we show the mass/charge distribution in the 40 to 100
amu mass range in Figure 15(d). It is evident that all charge states
of iron are well resolved.
The mass and mass/charge resolution capabilities of SWICS illustrated
above make it possible to identify He+ and obtain the energy spectra
of these ions. In Figure 16 we show the differential energy spectrum
of He+, summed over a seven day period. The sharp cut-off at
energy/charge corresponding to twice the solar wind speed measured
during this same period gives strong evidence that these are
interstellar pick-up ions such as first observed by Mobius et al.
(1985).
Acknowledgements.
The design, development, fabrication, testing and calibration of SWICS
would have been impossible without the ingenuity and dedicated efforts
of many individuals in the five cooperating institutions. We owe our
special thanks to Robert Cates (UMD), who designed portions of and
assembled the SWICS sensor, Karl Otto (MPAe), who debugged and tested
the compact analog electronics, H. Dinse (TUB), who programmed the DPU
and implemented the mass, mass/charge classification scheme, and
Charles Moyer (GSFC), who designed and built the BCE hardware. Dornier
System fabricated the flight and spare units of the analog electronics
and the DPU. For the calibration of the instrument, we are especially
grateful to Scott Lasley (UMD), Urs Schwab and Uli Rettenmund (Univ.
Bern), and Hartmut Sommer (MPAe). For assisting with the integration
of the engineering-model unit in the spacecraft and supporting
spacecraft test activities, we thank Uli Rettenmund (Univ. Bern). We
gratefully acknowledge the technical assistance and advice provided by
W. Frank, P. Caseley, H. Schaap, J.P. Bouchez and G. Tomaschek of
ESA/ESTEC. and by M. Agabra and J. Haas at JPL. Our special thanks go
to Tom Tomey, George Nickols, and Willis Meeks for their help and
encouragement during the time before launch when anomalies found in
both instruments had to be fixed.
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TABLE 1. Time-of-flight and energy-system characteristics for the
SWICS instrument.
------------------------------------------------------------------------------
Subassembly Characteristics
------------------------------------------------------------------------------
Time-of-flight assembly (TOF)
Flight path 10 cm
Start element carbon foil (3 [microns]/cm^2, grid
supported) (0.4x8 cm^2)
Stop element three rectangular solid-state
detectors (1.1 x 1.3 cm^2 active
each) Microchannel plates (MCP)
Type curved channel
Size (active) two rectangular (1.5 x 3.7 cm^2 each)
TOF telescope dispersion
Path length, [Delta]d/d ❮=0.005
Secondary electrons 0.3 ns
Energy measurement circuitry
Range 40-600 keV
Electronic noise (FWHM) 12 keV
E-ADC range 256 channels
E-ADC resolution 2.34 keV/channel
Time-of-flight measurement circuitry
Range 10-200 ns
Electronic noise(FWHM) 0.2 ns
T-ADC range 1024 channels
T-ADC resolution 0.195 ns/channel
------------------------------------------------------------------------------
TABLE 2. SWICS telemetry allocation for data items generated in the
analog electronics and DPU.
------------------------------------------------------------------------------
Data item Corresponding physical parameter
Name No. of Spin Origin
------------------------------------------------------------------------------
FSR 1 AE** start detector rate
DCR 1 AE start-stop coincidence rate (valid t
rate) TCR 1 AE valid t-solid-state detector (SSD)
coincidence rate
MSS 1 AE combined count rate of three main
channel SSDs
ACP 2 AE solar-wind proton rate***
ACA 2 AE solar-wind helium and heavy-ion rate
MEX 8 DPU 8 of 512 m vs m/q
matrix elements (see Fig. 10)****
MRX 18 DPU Counting rate of 18
selected ion species (see Fig. 10)*****
BRX 3 DPU 3 basic rates for each of 3
priority groups (see Fig. 10)
PHA****** 30 AE 30 24-bit E and T pulse-height events
ID 2 DPU ID synch word
HK 3 DPU Housekeeping data
------------------------------------------------------------------------------
[TABLE 2, cont.]
---------------------------------------------------------
Data item No. of bits per item
Name Per spin* Per second
---------------------------------------------------------
FSR 8 0.67
DCR 8 0.67
TCR 8 0.67
MSS 8 0.67
ACP 2 x 8 1.34
ACA 2 x 8 1.34
MEX 8 x 8 5.36
MRX 18 x 8 12.06
BRX 3 x 8 2.01
PHA****** 30 x 24 60.3
ID 2 x 8 1.34
HK 3 x 8 2.01
---------------------------------------------------------
1056 bit/format 88 bit/s
---------------------------------------------------------
* Voltage is stepped each spin = 12s. At spacecraft data rates lower
than 512 bit/s voltage is stepped at a slower rate.
** AE = analog electronics.
*** This rate is sectored into (1) a narrow (typically 45[degree] but
adjustable by command) sun-centered sector and (2) a broad
(typically 315[degree]) anti-solar sector.
**** Each of 512 matrix elements is accumulated in one complete voltage
cycle = 64 spins or 12.8 min. From these 512 matrix elements an
energy-averaged mass-mass/charge composition may be determined.
***** The 18 matrix rates correspond respectively to the following 18 ion
species: H^+ (E and T), ^4He^+2, H^+ (T, no E), ^4He^+ (T,
no E), singly ionised heavy ions (N^+, O^+, Ne^+ ...), C^+4.5, C^+6,
N^(+4 to 7), O^(+4 to 6), O^+7, O^+7, Ne(^+3 to 10), Mg^(+8 to 12),
Si(^+9 to 14), S(+5 to 16), Fe^(+6 to *) and Fe(+9 to 22).
****** 30 PHA events are telemetered in -12s in the spacecraft tracking-
mode transmission. 8 PHA events are telemetered in -12s in the
spacecraft storage mode transmission (512 bit/s).
TABLE 3. Mass and power for SWICS subsystems.
------------------------------------------------------------------------
Subsystem Mass(g) Power*(mW)
------------------------------------------------------------------------
-30kV supply (GLG-2B) 566 550
Sensor (GLG-2A) 3426 1900
Deflection system and deflection supply 1483 400
Analog electronics and bias supply 903 1200
Sensor elements and structure 1035 300
DPU (GLG-1) 1233 1500
DPU 1135 900
Low-voltage supply 120 600
Thermal blankets 300
Interconnect cable 56
------------------------------------------------------------------------
TOTAL 5584 g 3950 mW
------------------------------------------------------------------------
*Average power; peak power 4.8 W.
TABLE 4. E/Q, solar wind velocity ranges, and resolutions in the four
SWICS operating modes.
------------------------------------------------------------------------------
Mode-0 Mode-1* Mode-2 Mode-3
------------------------------------------------------------------------------
Energy charge range(keV/e)
Main channel 0.45-40 0.65-60 0.45-4.3 0.65-6.3
H/He channel 0.11-10 0.16-14 0.11-1.0 0.16-1.5
Velocity range (km/s)
Main channel
H 290-2800 350-3400 290-910 350-1100
He^++ 210-2000 250-2400 210-640 250-780
Fe^+8 110-1050 130-1280 110-340 130-420
H/He channel
H 150-1380 180-1640 150-440 180-540
He^++ 100-980 120-1160 100-310 120-380
Voltage range (V)
Main channel 47-4200 68-6300 47-450 68-660
H/He channel 7-670 11-945 7-67 11-100
Number of voltage
steps/cycle** 64 64 64 64
Voltage step range** 1-127(by 2's) 12-138(by 2's) 1-64 12-75
Step size (%) 7.44 7.44 3.65 3.65
------------------------------------------------------------------------------
* Normal mode of operation.
** Voltage is stepped each spin (12s). A voltage cycle consists of 64 steps
and is started at the highest voltage step, and then stepped down until
the counting rate in the start MCP exceed an value selectable by command
(typically 5 x 10^4 count/s). The voltage is then stepped up until the 64
steps are completed.
TABLE 5. SWICS resolution characteristics for typical solar-wind
ions*.
------------------------------------------------------------------------------
Energy** Time-of-flight [Delta](m/q)/(m/q) [Delta]m/m
Element Mass Charge (keV) (ns) (FWHM) (FWHM)
------------------------------------------------------------------------------
H 1 1 19 48.7 0.054 0.742
He 4 2 38 66.7 0.042 0.397
C 12 6 103 66.3 0.039 0.223
N 14 7 117 66.3 0.039 0.224
O 16 6 91 75.7 0.038 0.265
Ne 20 8 116 73.2 0.034 0.305
Si 28 9 122 80.6 0.033 0.302
S 32 10 133 81.6 0.033 0.305
Fe 56 11 111 98.9 0.030 0.353
------------------------------------------------------------------------------
* For 440 km/s solar-wind speed and 23kV post-acceleration
** Measured by sold-state detector
FIGURE 1. Schematic of the measurement technique used in the SWICS,
showing the functions of each of the five basic elements.
FIGURE 2. Functional block diagram for the SWICS experiment. The three
separately packaged units GLG-1 (DPU), GLG-2A (sensor) and GLG-2B
(post-acceleration supply) are shown as solid-border boxes.
Subsystems, and the responsible institutes, are indicated within each
of the three units. The flow of ions as well as signals, power and
control among the subsystems and units is also shown.
FIGURE 3. Cross-section of the SWICS sensor (GLG-2A) showing the
collimator, the two-channel deflection system and its deflection power
supply, the time-of-flight system and proton/helium detector, analog
electronics, sensor bias and power supply and opto-couplers for
digital data transmission. The three inner compartments are supported
by two bulkheads and are maintained at the post-acceleration voltage
(-15 kV to -30 kV). The outer diameter of the cylindrically shaped
outer housing is 15 cm.
FIGURE 4. The SWICS experiment.
FIGURE 5. Top view of the SWICS sensor, showing the cross-section of
the time-of-flight telescope, and the positions of the three solid-
state detectors, two microchannel plate assemblies, and the curved,
grid supported carbon foil. The shapes of the deflection plates and
the individual plates of the collimator are also shown.
FIGURE 6. Cross-section of the SWICS time-of-flight vs energy
telescope showing computer-generated trajectories of secondary
electrons emitted from the carbon foil and solid-state detector. The
front surface of each of the two curved-channel microchannel plates
(MCP) is biased sightly negatively with respect to the housing to
repel low-energy (❮100 eV) secondary electrons. A physical partition
between the two MCPs prevents secondary electrons from one MCP
triggering the other.
FIGURE 7. The SWICS time-of-flight vs energy telescope.
FIGURE 8. Functional block diagram of the SWICS analog electronics
consisting of four energy channels (preamplifier, shaping amplifiers,
discriminator) fed by the three TOF telescope solid-state detectors
(SSD) and the proton/helium SSD, a time-to-amplitude converter (TAC),
inflight calibration circuits providing energy and timing pulses, rate
logic and buffering, command logic, and time (T) and energy (E)
Analog-to-Digital Conversion (ADC) circuits. 24-bit pulse-
height/detector ID words and six rates are transmitted via opto-
coupler links to the Data Processing Unit (DPU).
FIGURE 9. Block diagram illustrating the operating principle of the
SWICS mass classification scheme. A 15 kbit ROM look-up table stores
coefficients for the Taylor expansion which are derived from SWICS
sensor calibration data. The most significant bits of the time and
energy pulse heights are used to select the three appropriate
coefficients from the table. Table-look-up multiplication and addition
using the three coefficients and the least-significant bits of the
pulse heights complete the computation of the ion mass.
FIGURE 10. Location of the 18 matrix-rate box boundaries (heavy
borders) in the m vs m/q plane. The "mass-zero" line corresponds to
ions (e.g. O^+) that had insufficient energy to trigger the solid-
state (energy) detector. The three priority range correspond to m ❮
8.7 or "mass-zero" with m/q ❮ 3.3 (Range-0), the shaded region with m
❯ 8.7 (Range-1), and the shaded region on the "mass-zero" line with
m/q ❯ 3.3. Dashed curves represent the expected locus of dominant
charge states for coronal temperatures of 2 x 10^6 K (left curve) and
10^6 K (right curve). Each if the boxed regions (both heavy and light
borders) is further divided into logarithmic evenly-spaced m/q bins of
3% in the Range 1 portion and 6% in the rest. These bins form a large
portion of the 512 matrix elements.
FIGURE 11. Calibration data for the flight microchannel plates that
were taken at the University of Bern acceleration facility in May
1990. Shown here are the relative efficiencies for an incident Neon
beams for the Double Coincidence Rate (D), the Triple Coincidence Rate
(T), and the Main Solid State Detector Rate (M) relative to the start
signal (Front Seda Rate, F). The ratios are plotted against total ion
kinetic energy/nucleon, after post acceleration. The rapid drop off at
low energies for the M/F and T/F ratios results from the 40 keV
threshold of the solid state detector.
FIGURE 12. Energy vs time color spectrogram of counts per voltage step
from the solid state detector of the H/He channel (top panel), and
from matrix rates recording primarily He^++, O^6+, and Si for days
347-349 (Dec 13-15) 1990. In these overview plots, changes in density
(color), bulk speed and kinetic temperature of representative solar
wind ions are easy to see. The double traces, seen best in the lower
two panels, are the result of the step reversal mode. (See text for
explanation).
FIGURE 13. Differential energy per charge spectra of H^+ and He^++
illustrating the ~ 10^8-10^9 dynamic range of the SWICS. A single
convected maxwellian, dj/dE = j[0]*exp [-(v-V)^2/(2kT/m)], fits the
peaks of both spectra, indicating that at this time solar wind protons
and alpha particles had the same bulk speed of 410 km/s and
temperature/mass of 13.5 x 10^4 K. A well-developed high energy, non-
maxwellian tail is observed in the spectra of H^+, and He^++.
FIGURE 14. Display of the mass vs mass per charge distributions of
solar wind ions derived from the raw energy and time-of-flight
pulseheight data collected by the SWICS during the 8 to 17 Dec, 1990
time period. The mass and mass/charge values were computed on ground
using algorithms identical to those employed by the instrument to do
on-board classification of solar wind ions. The color coded density
profiles (red: greater or equal to 30 counts/bin) show well-resolved
peaks of the major solar wind heavy elements and their dominant charge
states (e.g. C^6+, C^5+, O^7+, O^6+, Si^9+, Si^8+, Si^7+, Fe^11+,
Fe^10+, Fe^9+, Fe^8+). The relative abundances of the various charge
states of elements that can be derived from data such as shown here
may be used to derive the temperatures and temperature profiles of the
solar corona.
FIGURE 15. Examples of mass and mass per charge distributions
extracted from matrix element data [panel (a)] or from mass vs
mass/charge matrices (such as shown in Figure 14) [panels (b)-(d)]
with the SWICS at a post-acceleration voltage of 22.9 kV. These
distributions illustrate the mass and charge resolution capabilities
of the SWICS. We note that the mass and mass/charge resolution is the
same, regardless of solar wind flow conditions. The mass resolution,
however, will improve with higher post-acceleration voltage.
FIGURE 16. Differential energy flux spectrum of He+ ions observed on
May 27 June 3, 1991. The absolute flux values of this preliminary
spectrum may have systematic uncertainties of about a factor of two.
This spectrum is similar to that observed for He+ pick-up ions by
Moebius et al. (1985), with a sharp cut-off at twice the measured
solar wind speed of about 650 km/s.
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