Abstract
The Electron Drift Instrument (EDI) on the Magnetospheric Multiscale (MMS) mission measures the in-situ electric and magnetic fields using the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to the electric field and, to a lesser extent, the gradient in the magnetic field. Although these two quantities can be determined separately by use of different electron energies, for MMS regions of interest the magnetic field gradient contribution is negligible. As a by-product of the drift determination, the magnetic field strength and constraints on its direction are also determined. The present paper describes the scientific objectives, the experimental method, and the technical realization of the various elements of the instrument on MMS.
Highlights
The precise measurement of electric and magnetic fields in the rarefied plasmas within reconnection regions is the primary objective of the FIELDS suite on Magnetospheric Multiscale (MMS)
The sensor interfaces with the Gun/Detector electronics (GDE) which forwards commands received from the Electron Drift Instrument (EDI) Controller and returns event pulses and digital status to the GDE
The EDI processor transfers the programmed gun firing directions, plus the timing information from the correlators, to the necessary telemetry data so that the resultant drift-step direction and magnitude can be determined on the ground
Summary
The operational principle of the EDI technique was originally proposed by Melzner et al (1978) and proven on ESA’s GEOS spacecraft. This instrument was limited to spin period time resolution and could be operated only for small angles between the magnetic field vector and the spacecraft spin axis. The generation of EDI instruments on Equator-S and Cluster (see Paschmann et al 1998) could determine the electric field for any direction of the ambient magnetic field and was shown to be highly accurate even in the presence of photoelectron and wake asymmetries that often occur around spacecraft in rarefied plasmas.
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