Understanding the Driver of Energetic Electron Precipitation Using Coordinated Multisatellite Measurements

Abstract

Magnetospheric plasma waves play a significant role in ring current and radiation belt dynamics, leading to pitch angle scattering loss and/or stochastic acceleration of the particles. During a non‐storm time dropout event on 24 September 2013, intense electromagnetic ion cyclotron (EMIC) waves were detected by Van Allen Probe A (Radiation Belt Storm Probes‐A). We quantitatively analyze a conjunction event when Van Allen Probe A was located approximately along the same magnetic field line as MetOp‐01, which detected simultaneous precipitation of >30 keV protons and energetic electrons over an unexpectedly broad energy range (>~30 keV). Multipoint observations together with quasi‐linear theory provide direct evidence that the observed electron precipitation at higher energy (>~700 keV) is primarily driven by EMIC waves. However, the newly observed feature of the simultaneous electron precipitation extending down to ~30 keV is not supported by existing theories and raises an interesting question on whether EMIC waves can scatter such low‐energy electrons.

Direct Observation of Subrelativistic Electron Precipitation Potentially Driven by EMIC Waves

“…While precipitating protons can produce proton aurora Yuan et al, 2010),~MeV electrons can interact with atmospheric nitrogen and hydrogen oxides, leading to ozone reduction (e.g., Meraner & Schmidt, 2018). The efficacy of EMIC waves in scattering MeV electrons and ring current protons has been confirmed in multiple studies, both observationally and theoretically (Blum et al, 2015;Capannolo et al, 2018Capannolo et al, , 2019Hirai et al, 2018;Qin et al, 2018Qin et al, , 2019Shekhar et al, 2017;Woodger et al, 2018;Yuan et al, 2018). However, the techniques and the satellites used have limitations that still leave a few open questions, for example, on the minimum energy of electrons (E min ) that can be scattered into the loss cone.…”

Precipitation of Radiation Belt Electrons by EMIC Waves With Conjugated Observations of NOAA and Van Allen Satellites

“…In addition, in the presence of sufficiently narrow cold plasma structure, the precipitation regions of radiation belt electrons related to different wave modes should nearly coincide so that it is difficult to attribute precipitation of radiation belt electrons only to EMIC waves (Yahnin et al, ). Recently, some studies have focused on the link between EMIC waves observed by the inner magnetospheric satellites and the precipitation of radiation belt electrons detected by the ionospheric satellite or inferred from the ground‐based observations (e.g., Blum et al, ; Capannolo et al, ; Li et al, ; Qin et al, ; Rodger et al, ; Usanova et al, ; J. Zhang, Halford, et al, ). However, the precipitations of >MeV radiation belt electrons always coincided with the precipitations of <MeV RC electrons, implying it difficult to attribute precipitation of radiation belt electrons only to EMIC waves.…”

“…Recently, some studies have focused on the link between EMIC waves observed by the inner magnetospheric satellites and the precipitation of radiation belt electrons detected by the ionospheric satellite or inferred from the ground-based observations (e.g., Blum et al, 2015;Capannolo et al, 2018;Li et al, 2014;Qin et al, 2018;Rodger et al, 2015;Usanova et al, 2014;J. Zhang, Halford, et al, 2016).…”

Precipitation of MeV and Sub‐MeV Electrons Due to Combined Effects of EMIC and ULF Waves

“…During the conjugate observations between Van Allen Probes and POES, significant and sporadic increases of precipitating (field‐aligned) electron fluxes measured by POES can be seen in Figure c after 11:02 UT in the ≈300‐ to 700‐keV energy range, sometimes reaching the level of the trapped fluxes (at 90°). Such sub‐MeV electrons could have been affected by resonant wave‐particle interactions and precipitated into the atmosphere, possibly by the combined effects of EMIC and ULF waves (see similar observations from Capannolo et al, ). Simultaneously with EMIC wave observations, some decrease of field‐aligned electron (0.5–1.5 MeV) fluxes was observed by the Van Allen Probes near the equator (not shown): It could also be due to a precipitation induced by EMIC waves (e.g., see Usanova et al, ; Wang et al, ).…”