Instrument Overview
===================
 The Ion Composition Instrument is a novel spectrometer for measuring
 the ionic composition of the solar wind from the ICE (ISEE-C, ISEE3)
 spacecraft.  The resolution and dynamic range of the instrument are
 sufficient to be able to resolve up to twelve individual ions or groups
 of ions.  This will permit the solution of a number of fundamental
 problems related to solar abundances and the formation of the solar
 wind.  The spectrometer is composed of a stigmatic Wien filter and
 hemispherical electrostatic energy analyzer.  The use of curved
 electric field plates in the filter results in a substantial saving of
 weight with respect to a conventional filter of the same resolution and
 angular acceptance.
 
 The spectrometer is controlled by a microprocessor based on a special
 purpose computer which has three modes of operations: full and partial
 survey modes and a search mode.  In the search mode, the instrument
 locks on to the solar wind.  This allows four times the time resolution
 of the full survey mode and yields a full mass spectrum every 12.6 min.
 
SCIENTIFIC OVERVIEW
===================
 
 ION COMPOSITION studies in the solar wind are essential for
 understanding the dynamics and energetics of the solar wind
 acceleration region, and are an important source of solar abundance
 information.  The ion composition of the solar wind is, however, poorly
 known because of the technical difficulties associated with the
 separation of the different ions over the wide dynamic range involved.
 
 With electrostatic energy analyzers, the He+2 /H+ abundance ratio has
 been studied extensively in the solar wind, and large variations found.
 During quiet low-temperature conditions, ions other than H+ and 4He+2
 have been separated with this technique and the presence of heavy ions
 has been demonstrated.  In addition, some information about abundances
 and charge state distributions has been obtained.
 
 The foil collection technique has been used to determine the abundances
 of the isotopes of He, Ne, and Ar in the solar wind. Although this
 method has yielded precise abundance data, its time resolution is
 limited.
 
 The ICE spacecraft will be continuously in the solar wind and thus
 affords an opportunity for a comprehensive uninterrupted composition
 study.  With a continuous record of such a large number of ions,
 fundamental problems concerning the origin of the solar wind and solar
 composition can be investigated. Some of these are discussed below.
 
 1) A number of momentum transfer mechanisms for the acceleration of
 heavy ions into the solar wind have been proposed.  Under the inferred
 conditions in the solar wind source region, coulomb collisions would be
 sufficient to transfer momentum from protons to heavy ions with high
 Z**2/M ratios.  Momentum transfer by waves has also been proposed as an
 efficient mechanism.
 
 To assess the relative importance of the various mechanisms,
 measurements of solar wind ions over a large range of M and Z under
 varying conditions in the corona and solar wind are necessary.
 
 2) Bame et al. (1978) have recently proposed that the He/H ratio of
 0.04-0.05 typically found in fast solar wind streamers be equated with
 the He/H ratio in the outer convective zone of the sun.  This is a
 factor of two lower than the generally accepted value and, if
 correct, would reduce the calculated boron neutrino flux.  A
 comprehensive investigation of the ion abundance variations should lead
 to a better understanding of ion fractionation processes in the solar
 corona, and in turn should yield accurate estimates of the He/H ratio.
 
 3) A correlation between the He/H ratio and solar wind speed has been
 observed but the mechanism is unknown. By extending correlation studies
 to other ions it may be possible to uncover the mechanism.
 
 4) Local temperatures and temperature gradients in the corona can be
 estimated from measurement of the charge distribution of Fe ions and
 the charge states of 0 and Si. To test the validity of the various
 solar wind expansion models, the experimentally derived temperatures
 and gradients can be compared to those derived from the models.
 
 5) An average 4He/3He ratio of 2350 +- 120 has been derived from the
 five Apollo solar wind foil experiments , however, the ratio appears to
 be highly variable.  In one instance, a ratio of 500 was observed over
 a two-day period With the mass spectrometer on ICE (ISEE-C), 3He will
 be continuously monitored, permitting an accurate determination of the
 solar surface abundance of this cosmologically important nucleus. The
 relation between anomalous 3He abundances in the solar wind and the
 small 4 He/3He ratios in some solar flares can also be studied.
 
 
Calibration
===========
 The sensor was calibrated at the test facility of the Physikalisches
 Institut of the University of Bern.  This facility consists of a
 vacuum chamber in which a homogeneous monoenergetic mass separated
 beam of ions can be produced and directed at the sensor mounted on a
 test platform which can be rotated about two axes perpendicular to the
 direction of the ion beam.  Sweep circuits were used to synchronously
 change the voltages on the plates of the filter and analyzer while the
 outputs of each of the three CEM's strobed a multi-channel analyzer
 operating in the pulse-height-analysis mode. Calibrations were
 performed with H2+ and He+ at nominal velocities of 300, 400, 500, and
 600 km/s.  The orientation of the sensor with respect to the incident
 ion beam was changed in small increments over a range of +- 8 degree in
 the horizontal plane and +- 14 degree in the vertical plane.  Angular
 acceptances derived from these functions are in good agreement with
 theory, whereas the experimental resolution always exceeds that
 predicted. The absolute values of the transmission determined at normal
 incidence for H2+ and 4He+ at the four different values of ion velocity
 are in excellent agreement with theory.  The ability of the instrument
 to reject stray ions which create ghost peaks by scattering from the
 outside plates of the analyzer and contribute to the background was
 measured.  In all cases, the ratio of ghost peak counts to counts in
 the main peak was less than 10**-4  with the ghost peaks always
 appearing at energy values 7 percent less than those of the
 corresponding main peaks. Because of the large difference in M/Z
 between 4He and 3He, interference of 4He+2 at the position of the 3He+2
 peak is less than 10**-5 of the 4He signal.
 
 
Operational Considerations
==========================
 The input voltages to the power supplies which provide the potential to
 the electric field plates of the Wien filter and energy analyzer are
 obtained from 12-bit digital-to-analog converters.  The digital
 inputs to the converters come from the microcomputer-based processor.
 The full parameter range covered by the experiment is 300-600 km/s in
 velocity, 840- 11720 eV/Z in energy per charge, and 1.5-5.6 in mass per
 charge.  The velocity range is divided into n steps and the energy
 per charge range into m steps, with both being logarithmically
 related.
 
     V(1s), (n, m) = R1**(m-1)*R2**(n-1)*V1(1,1),    energy analyzer
 
        V(2s),(n) = R2** (n -1)*V2 (1),  Wien filter
 
 1 < n < 24, 1