| DESCRIPTION |
Instrument Overview
===================
The ISEE-1 and ISEE-C instruments have been designed to measure the
elemental abundances, charge state composition, energy spectra, and
angular distributions of energetic ions in the energy range 2
keV/charge to 80 MeV/nucleon and of electrons between 75 and 1300 keV.
By covering the energy range between solar wind and low-energy cosmic
rays the instrument will fill a gap in the knowledge especially of the
nuclear and ionic composition of solar, interplanetary, and magneto-
spheric accelerated and trapped particles. The instrument consists of
three different sensor systems: ULECA is an electrostatic deflection
analyzer system with rectangular solid-state detectors as energy deter-
mining devices, its energy range is -3 to 560 keV/charge; the ULEWAT is
a double dE/dX versus E thin-window flow-through proportional
counter/solid-state detector telescope covering the energy range from
0.2 to 80 MeV/nucleon (Fe); the ULEZEQ sensor consists of a com-
bination of an electrostatic deflection analyzer and a thin-window
dE/dX versus E system with a thin-window proportional counter and a
position-sensitive solid-state detector. The energy range is 0.4
MeV/nucleon to 6 MeV/nucleon. While the ULECA and the ULEWAT sensors
are designed mainly for interplanetary and outer magnetospheric
studies, the ULEZEQ sensor will also obtain composition data in the
trapped radiation zone. 65 rates and pulse-height data can be
obtained with sectoring in up to 16 sectors.
Scientific Objectives
=====================
The low-energy portion of interplanetary and magnetospheric particles
(energies 1 keV/nucleon to several hundred keV/nucleon) has been
virtually unexplored to date. Measure. ments in this region, however,
are expected to provide sensitive probes of solar, interplanetary, and
magnetospheric phenomena.
A. SolarandlnterplanetaryPhenomena
1) Solar Flare Acceleration Mechanism: The mechanism by which ions are
accelerated in flares is not presently known. Important information on
this mechanism, however, is avafl. able from studies of the energy
spectra of the particles which the mechanism produces. This is
particularly true in the case of heavier ions at low energies. These
particles are not expected to be fully stripped of their electrons.
Thus in the same energy range different ions cover different rigidity
in ter-vals. By examining, then, the energy spectra of many ions at
low energies, and also electrons, we will place stringent constraints
on the rigidity dependence of the flare acceleration process.
2) The Location of the Flare Acceleration Region: Low-energy ions also
provide in their charge states infon-nation on the region in the corona
where they are accelerated. The charge of solar particles is frozen-in
as the particles leave the corona; the plasma density in interplanetary
space is too low to cause additional ionization or recombination. Thus
the charge composition, particularly of very low energy ions, can
reflect the temperature of coronal electrons in the region where the
particles are accelerated, propagate, or are stored, For example, if
the ions are accelerated in the flare site itself, we should expect a
high degree of ionization, appropriate to the high temperature in the
flare. In contrast, if the particles are accelerated in the
surrounding corona by, e.g., plasma disturbances emitted from the
flare, we should expect a charge composition more similar to that of
the solar wind.
3) Coronal Propagation and Storage: Low-energy ions also provide some
of the most detailed information on coronal propagation mechanisms, and
coronal storage. Measurements in the broad rigidity range covered by
partially stripped ions at low energies, and also by low-energy
electrons, will provide stringent tests of the current idea that
coronal propagation is rigidity-independent. Further, ionization
loss which is a consequence of extended coronal storage is most evident
at low energies, where it should produce in the differential energy
spectra a systematic flattening which depends on the charge squared to
mass ratio. From the flattening observed in the spectra of different
ions, it should be possible to determine the time during which the
particles are stored and/or the coronal density and thus the location
of the region where the storage occurs.
4) Compositional Variations in Solar Flares: The composition of
energetic flare particles is known to vary widely from flare to flare,
particularly at low energies. The cause of this variability is not
presently known. It may result from the fact that heavier ions, which
may not be fully stripped of their electrons, can exhibit different
rigidities in different flares, and thus will respond differently to
the flare acceleration process. It may be also that the variations
reflect compositional anomalies in the coronal material from which
the particles are accelerated. Our investigation, which can deter-
mine the charge states of low-energy ions, will probe the former
explanation in detail. From these charge-state measurements, which
can indicate the coronal conditions where the particles are
accelerated, and from our general ability to observe flares in
considerable detail, we will also provide information on the latter
possibility.
Of particular interest in the study of compositional variations in
solar events are the so-called 3He-rich event, in which the 3He/4He
ratio can exceed unity. These flares are one of the more dramatic
examples of compositional anomalies since the coronal abundance is
3He/4He < 10**-3 . These flares also have the peculiarity that there
is no accompanying increase in deuterium and tritium, which are equal
byproducts of 3He in spallation reactions, and there is a general
enhancement in heavier elements, particularly in iron. Our
investigation will extend the measurements of 3 He and heavier ions in
these flares to much lower energies than has been possible to date.
5) Interplanetary Propagation: The current theory for energetic
particle interactions with the magnetic field in the solar wind is
inadequate in that it predicts more pitch-angle scattering than is
observed. These differences are most pronounced, and thus most easily
studied at low energies (< 1 MeV/nucleon). We will make a detailed
study of propagation at low energies by observing the anisotropies of
protons and helium throughout our energy range, as well as the time
profiles of different ion species and electrons. The former
measurement is a sensitive indicator of the extent of the scattering;
both measurements provide information on its rigidity dependence.
6) Interplanetary Acceleration: It appears from studies in recent years
that the majority of co-rotating particle streams are the result of
interplanetary acceleration, as opposed to of direct solar origin. The
origin of the particles, however, is not presently known. It may be
that the particles are accelerated out of the solar wind. It is
possible also that the particles enter the solar wind as energetic
solar particles and receive here an additional acceleration. Our
measurements of the spectra, anisotropies, and charge states of ions,
over an essentially continuous energy range from the solar wind to
high-energy particles, should provide stringent tests of these pos-
sible origins. If these particles are accelerated out of the solar
wind, their charge states should be those of solar wind ions, and their
spectra a continuous extension of the solar wind distribution.
Particles which originate as more energetic solar particles, in
contrast, may be more highly ionized and may exhibit a behavior at
higher energies which is uncorrelated with that closer to solar wind
energies.
7) The Anomalous Cosmic-Ray Component: From 1972 on, a component with
the anomalous composition of only helium, nitrogen, oxygen, and neon
has been observed in the cosmic-ray flux at energies 1-30 MeV/nucleon.
The origin of this component is presently being debated, although there
is mounting evidence that it results from interstellar neutral
particles which are swept into the heliosphere and ionized and
accelerated in the solar wind. This component appears to be a feature
of solar minimum conditions. Our observations over the next few years
will record the behavior of this component as solar activity increases
with the onset of the new solar cycle.
8) Correlated Studies with Deep-Space Missions: Our investigation
will also provide measurements at earth for correlated studies of
galactic cosmic-ray modulation and solar flare propagation with deep-
space missions such as Pioneer and Voyager.
Detectors
=========
The ULECA (Utra Low Energy Charge Analyzer) sensor incorporates
techniques of electrostatic deflection and a total energy measurement
to provide the charge composition of ions in the energy range 2 to
560 keV/charge. ULECA is a design of the University of Maryland ECA
(Energy Charge Analyzer) instrument flown on IMP 7 and 8 satellites .
Low-energy ions pass through a multisht focusing collimator and enter
one of three deflection regions designed by L, M, and MP. Seven
rectangular surface-barrier (Au-Si) solid-state detectors are placed at
fixed positions at the exit of the deflection regions, each defining
a given energy per charge window. The output of each of these
detectors is pulse-height analyzed provided the solid-state
anticoincidence detectors are not triggered. A residual background
(due to cosmic-ray produced secondaries) and the geometrical factor of
the collimator determine the minimum flux which can be measured by each
detector (see table below). The majority of penetrating particles is
eliminated from analysis by the solid-state anticoincidence detectors.
TABLE
RATE CHANNEL CHARACTERISTICS ULECA SENSOR
Rate Readout
Designation Particle Energy Range Period Conversion Factor Minimum Flux
(N/S) 1 Type 2 (keV/charge) (S) 3 CM**2-sr-keV CM**2-sr-s-keV
------------ --------------
charge**-l charge**-l
Ll (S) > 3 2.88-3.084 16 581 290
8.68-9.29 191 95
L2 (S) Q > 1 8.14-9.384 16 94 47
26.4-30.5 28 14
Mll (N) Q = 1 27.4-32.7 8 18.5 1520
(S) 32 5
M12 (N) Q = 2 27.4-32.7 8 18.5 8
(S) 32 3
M13 (N) Q > 2 27.4-32.7 32 18.5 61
(S) 64 15
M21 (N) Q = 1 53-69 8 5.35 17
(S) 64 1.3
M22 (N) Q = 2 53-69 8 5.35 6
32 0.7
M23 (N) Q > 2 53-69 64 5.35 2.2
M31 (N) = 1 103-140 8 2.52 298
32 0.7
M32 (N) Q = 2 103-140 8 2.52 0.3
M33 (N) Q > 2 103-140 32 2.52 0.2
MP12(N) 1,2 105-560 64 0.33 3.5
MP3L 3 105-560 64 0.33 0.2
MPI L = 1 105-560 32 0.33 3.3
MP2L Q = 2 105-560 32 0.33 0.1
1 N = nonsectored; S sectored into 8-45 deg sectors in ecliptic
plane.
2 Q = charge state of ion.
3 At high bit rate divide period by 4.
4 L1, L2 energy range assumes medium voltage mode.
The major effect not included is the secondary electron background in
the solid-state detector. This effect is caused by the interaction of
high-energy particles in the spacecraft. Our experience with similar
detector systems on IMP's 7 and 8 indicate that this will not be an
important effect for magnetospheric events.
In the L deflection region, a variable power supply steps the
deflection voltage in 32 logarithmic increments from 600 to 1550 V,
stepping once every 5 spin periods (16 s). The voltage range may be
changed up or down by 50 percent via ground con-unand. Two low-noise
(1 5-keV energy threshold) rectangular solid-state detectors Ll and L2
measure the energy and record the counting rate of deflected ions for
each voltage step. The relative energy per charge windows delta E/E
are 0.07 and 0.15 (FWHM), and the energy ranges over which measurements
are made are 1.8 to 9.5 and 5.1 to 30.5 keV/charge for L1 and L2,
respectively.
In the M and MP deflection region, two high-voltage supplies are used
to provide deflection fields of 1.5 and 6.7 kV/cm, respectively. At
the exit of the M section 3 rectangular solid-state detectors, Ml, M2,
and M3 measure the fluxes and anisotropies of protons, alpha's and Q >
4 ions in the energy range 25 to 140 keV/charge. These detectors are
also pulse-height-analyzed for more detailed charge spectra. In
the MP section, a single position-sensitive rectangular Si detector is
used to determine the charge spectrum of ions (H to Fe) from 105 to 560
keV/charge.
Measured Parameters
===================
Each detector is monitored by one or more threshold discriminators
whose outputs are passed through a system of coincidence-
anticoincidence logic which generates 65 different counting rates
corresponding to a variety of detector combinations, particle energy
windows, etc. The number of accumulated counts for any particular rate
is stored in one 24-bit register in a 256 register memory. (There are
two such memories, identical in all respects, for redundancy. For the
most part only one memory is used with the second remaining as a
backup, with data storage able to be switched between the memories by
command.) Individual rates are read out at predetermined intervals and
logarithmically compressed to either 10 or 12 bits and inserted into
the telemetry stream. Some rates are accumulated continuously over all
directions during the spacecraft spin (omnidirectional rates), others
are sorted into 8 directional angular sectors so as to measure flux
anisotropies (sectored rates). Of these rates, the BASIC rates are of
special importance. Their logic requirements are identical to the
ones required to trigger individual pulse-height analyzed (PHA) events.
Absolute fluxes can therefore, be computed from PHA events. Due to the
large number of rate channels generated, the telemetry of individual
rates involves a rather complicated submultiplexing scheme, which
cannot be described here.
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