Ion composition measurement techniques for space plasmas

Abstract

Plasmas found in space range from the solar wind with a typical temperature of 105–106 K, ∼400 km/s bulk flow speed, and high ionization (charge states) of ions, to the hot, slowly moving plasmas in the outer magnetospheres of the giant planets, to the cold, co-rotating plasmas in inner magnetospheres. While measurements of the distribution functions of protons and electrons, which have been available for several decades of space exploration, are providing important information about the heating and acceleration mechanisms producing these plasmas, knowledge of the elemental composition and charge states of ions in these plasmas is essential for establishing their sources of origin. We review space plasma instruments and techniques, with an emphasis on hot plasma composition measurements. Starting with Faraday Cup detectors some 30 years ago, plasma instruments have evolved to the present time-of-flight systems with excellent mass resolution and three-dimensional viewing capabilities.