Summary

The Comprehensive Plasma Instrumentation (CPI) for the Geotail spacecraft consists of three plasma analyzers: (1) an electrostatic analyzer for hot electrons and ions, (2) a complementary electrostatic analyzer for cool plasmas with high bulk speeds such as those found within the solar wind and magnetosheath, and (3) an ion composition analyzer for identification of ion species such as H⁺, He⁺, He⁺⁺, and O⁺. The energy-per-unit charge (E/Q) ranges for these analyzers are 1.3 V to 48.2 kV, 145 V to 6830 V, and 1.3 V to 48.2 kV, respectively. Three-dimensional velocity distributions of the ions and electrons are acquired for the first time in the distant magnetotail and are sufficiently accurate to separate E × B drifts from field-aligned flows associated with currents. Among the many specific scientific objectives of the plasma observations are the relative importance of ionospheric and solar wind sources, searches for plasmoids and flux ropes, current-sheet acceleration of plasmas, and convection and current systems in the magnetotail.