Reference: Moore, T.E., C. R. Chappell, M. O. Chandler, S. A. Fields, C. J. Pollock, D. L. Reasoner, D. T. Young, J. L. Burch, N. Eaker, J. H. Waite, Jr., D. J. McComas, J. E. Nordholt, M. F. Thomsen, J. J. Berthelier, and R. Robson, The Thermal Ion Dynamics Experiment and Plasma Source Instrument, Space Sci. Rev., 1995.
Introduction:
The Thermal Ion Dynamics Experiment/Plasma Source Instrument derives from a broad range of experience with low-energy plasma measurements at high altitude extending from some of the earliest measurements of the plasmasphere on OGO-5, including the ATS-6, GEOS, SCATHA, and ISEE plasma instruments, and culminating most recently with the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer 1 (DE 1), which operated between September 1981 and February 1991 [Chappell et al., 1981; Chappell, 1988]. These instruments measured angular and energy characteristics by mass species, revealing that non-thermal features often characterize the "thermal plasma," and leading to a new appreciation of the three-dimensionality of plasma flows, including field-aligned bulk motions of the ionosphere.
Two fundamental lessons were learned from these earlier experiences: (1) high-altitude spacecraft carry plasma instruments into regions of very low plasma density where the range of geometric factors (combined area and solid angle response) previously considered adequate for plasma instrumentation (e.g., ~10-2 cm2 sr), becomes inadequate; (2) positive floating potentials of spacecraft exposed to sunlight and low-density plasmas make low-energy plasma observations all but impossible by excluding low-energy ions from the spacecraft, exacerbating the sensitivity problem. Experience with the ATS-6 and SCATHA spacecraft [Olsen, 1981, 1985] has shown that spacecraft floating potential can be regulated at small values by means of the operation of a low-energy plasma source on the spacecraft. It should be noted that both of these problems are peculiar to low-energy plasma measurements, because the flux of hot plasma particles is relatively unaffected by floating potential effects and is larger than for a similar density of low-energy particles. Consequently, hot plasma ions are more readily observable than an equal density of very low-energy ions.
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