PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ICE INSTRUMENT_ID = ICI OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ION COMPOSITION INSTRUMENT" INSTRUMENT_TYPE = "ION MASS SPECTROMETER" INSTRUMENT_DESC = " 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 funda- mental 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 ac- cepted value and, if correct, would reduce the calculated boron neutrino flux. A comprehensive investigation of the ion abundance variations should lead to a better under- standing 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 im- portant nucleus. The relation between anomalous 3He abun- dances 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 Physi- kalisches Institut of the University of Bern. This facility con- sists of a vacuum chamber in which a homogeneous mono- energetic 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 contrib- ute 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 en- ergy analyzer are obtained from 12-bit digital-to-analog con- verters. 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 en- energy per charge range into m steps, with both being loga- rithmically 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