Drift‐Dispersed Flux Dropouts of Energetic Electrons Observed in Earth's Middle Magnetosphere by the Magnetospheric Multiscale (MMS) Mission

Abstract

Energetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of “flux‐reduced injections” resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection.

Drifting Electron Holes Occurring During Geomagnetically Quiet Times: BD‐IES Observations

“…Therefore, the DEHs mainly originated from the postmidnight magnetosphere. This result agrees with Cohen et al ().…”

“…The flux dropout reported by Cohen et al () is different from the usual concept of flux dropout (e.g., Turner, Morley, et al, ; Yuan & Zong, ). The usual dropout occurs more often above 1 MeV and generally very strong (almost full dropout), whereas the dropout reported by Cohen et al () is limited below ∼500 keV, and its amplitude is weak. These differences imply that the two types of dropout are caused by different mechanisms.…”

“…It then arises a question of whether the quiet time activity could also induce particle flux dropout. Recently, with Magnetospheric Multiscale (MMS) spacecraft, Cohen et al (2019) reported that flux dropout of several hundred keV electrons could occur during geomagnetically quiet time. The flux dropout reported by Cohen et al (2019) is different from the usual 10.1029/2019JA027194 concept of flux dropout (e.g., Turner, Morley, et al, 2012;Yuan & Zong, 2013).…”

“…Recently, with Magnetospheric Multiscale (MMS) spacecraft, Cohen et al (2019) reported that flux dropout of several hundred keV electrons could occur during geomagnetically quiet time. The flux dropout reported by Cohen et al (2019) is different from the usual 10.1029/2019JA027194 concept of flux dropout (e.g., Turner, Morley, et al, 2012;Yuan & Zong, 2013). The usual dropout occurs more often above 1 MeV and generally very strong (almost full dropout), whereas the dropout reported by Cohen et al (2019) is limited below ∼500 keV, and its amplitude is weak.…”

“…These differences imply that the two types of dropout are caused by different mechanisms. Cohen et al () proposed an interpretation scenario which is similar to the one for substorm time DEHs (Sergeev et al, ). However, due to limitation in observations (e.g., the lack of coordinated observations in the magnetotail), this quiet time electron dropout is not yet fully understood.…”