The Phase Relation Among Microinjection, Thermal Plasma, and Electric Field: A Case Study

Abstract Microinjections are periodic enhancements of 10 s–100 s keV electron fluxes at a cadence of several minutes on the dawnside or duskside magnetosphere. Here, we extend previous studies by comprehensively examining the plasma and wave observations provided by the Magnetospheric Multiscale (MMS) mission. We report an event study showing that microinjection oscillates in specific phase relations relative to lower‐energy (1 keV) plasmas (180° out of phase) and the electromagnetic waves and the corresponding E × B velocity (90° out of phase). The phase relations can be explained by the following simple scenario: A plasma boundary, which separates the low‐energy plasmas and the plasma accessing the 10 s–100 s keV electrons, oscillates over the quasi‐stationary MMS satellites due to the Ultra‐Low Frequency (ULF) waves. The phase relations and the simple explanation suggest that the period of microinjection may be apparent, that is, the 10 s–100 s keV electron flux could be stable in the rest frame of its local plasma.