In Situ Measurements of Plasma Convection with FAST TEAMS

Abstract:

The measurement of cold plasma in situ by traditional particle instruments is usually complicated by issues such as positive spacecraft charging, spacecraft motion, and low energy resolution in the range of interest. The Fast Auroral SnapshoT (FAST) spacecraft, however, was observed on orbit to have a relatively low charge most of the time and a tendency to charge negatively rather than positively. FAST also carried the Time-of-flight Energy Angle Mass Spectrometer (TEAMS), which measured H⁺, He⁺, and O⁺ distributions over the energy range 1-12000 eV. FAST's orbital motion shifts the cold background O⁺ population into the energy range of TEAMS, which has sufficient energy and angular resolution to measure the difference between the O⁺ ram velocity and the spacecraft velocity. This difference is due to convection of the plasma. We have developed a technique to measure this velocity. Since our technique does not rely on electric field measurements, we can compute 2-D convection velocities without depending on axial electric field measurements. We present examples of the measurement and discuss methods for validating the measurement against other techniques, both in situ and ground-based.