The Plasma Instrument for AMPTE IRM

The AMPTE IRM plasma instrument package consists of three sensors. Two of them measure complete 3-D velocity-distribution functions of ions and electrons every spacecraft revolution (i.e., 4.35 s). The third sensor is a retarding potential analyzer (RPA) for low-energy electron measurements. The 3-D measurements consist of countrates at 30 energies and 128 angles evenly distributed over the 4xr solid-angle sphere. The energy range is 15 eV-30 keV for electrons and 20 eV/q-40 keV/q for ions. The RPA extends the electron measurements to lower energies. Three microcomputers within the experinent perform extensive on-board data-processing functions. Two of them compute the moments (density, velocity, temperature tensor, and heat flux vector) of the distribution functions of ions and electrons in real time. The third compresses the RPA data and computes an indication for the spacecraft potential.