Drift‐Dispersed Flux Dropouts of Energetic Electrons Observed in Earth's Middle Magnetosphere by the Magnetospheric Multiscale (MMS) Mission. Issue 6 (28th March 2019)

• Record Type: Journal Article
• Authors: Cohen, Ian J.; Mauk, Barry H.; Turner, Drew L.; Fennell, Joseph F.; Blake, J. Bernard; Reeves, Geoffrey D.; Leonard, Trevor W.; Baker, Daniel N.; Jaynes, Allison N.; Spence, Harlan E.; Gabrielse, Christine
• Is Part Of: Geophysical research letters. Volume 46:Issue 6(2019)
• Page Start: 3069
• Page End: 3078
• Publication Date: 2019-03-28
• Subjects: MMS -- injections -- magnetosphere; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL082008
• Languages: English
• ISSNs: 0094-8276

Abstract

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. Plain Language Summary: Energetic particles are suddenly and abruptly pushed earthward through near‐Earth space via processes called "injections." These are commonly seen by spacecraft as sudden time‐ and energy‐dependent increases in the number of observed particles and are known to play an important role in sourcing the planet's radiation belts. Investigating the processes governing energetic particle transport is critical to better understand fundamental plasma processes, the dynamics of Earth's radiation belts, and the effects of space weather. This paper investigates a set of similar, but unique, signatures where the number of particles suddenly decreases instead of increasing. The analysis presented here surprisingly reveals that the particles observed in these signatures come from the magnetotail, which raises new questions about what generates these signatures. We investigate two hypotheses to explain the observed decrease in particles: (1) that a process removes the particles from the system and (2) that a process similar to "normal" injections moves a population of decreased particles into an area where there are more particles (hence generating a local decrease). The analysis finds that the secondary hypothesis is far more likely to be the primary driver while the first cannot be ruled out entirely as a secondary contributor. Key Points: MMS/FEEPS has repeatedly observed energy/time‐dispersed transient reductions in energetic electron flux in the dawnside magnetosphere Drift tracing consistently indicates that these signatures are from a postmidnight magnetotail source Results suggest that these features result from earthward injection in the presence of a negative radial gradient in phase space density