The Fast Plasma Instrument for the MMS Mission

A clear picture of short-scale reconnection structures and their rapid motions will require observations from closely spaced platforms at a 30ms measurement cadence. The Fast Plasma Instrument (FPI) for the MMS mission exceeds this demanding requirement by acquiring full sky, high-resolution (11deg) electron plasma velocity distributions every 25 ms. FPI also delivers four full sky, medium-resolution (45deg) distributions every 6 ms, for unprecedented access to electron scale dynamics within the reconnection diffusion region. Data compression and burst memory management provide up to 16 minutes of high time resolution data during each orbit of the four MMS spacecraft. Each spacecraft will intelligently downlink the data sequences that contain the greatest amount of temporal structure. For both electrons and ions, FPI will realize these specifications by means of eight half-top-hat energy analyzers. Each analyzer has a 180-deg x 6-deg fan-shaped field of view (FOV) aligned with the s/c spin axis, and is fitted with lateral FOV deflection electrodes. The analyzers are packaged as four Dual Electron Spectrometers and four Dual Ion Spectrometers on each spacecraft. When distributed properly around the spacecraft, these packages provide an instantaneous full-sky view that is independent of spacecraft spin rate. This approach makes available a very large instantaneous aperture for plasma measurements at the high sensitivity required for fast exposures. FPI is based on flight heritage from Cluster/PEACE, Geotail/LEP, Polar/Hydra, and Rosetta/IES.