On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study*

Abstract

Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron-Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L<5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

Dynamic Mechanisms Associated With High‐Energy Electron Flux Dropout in the Earth's Outer Radiation Belt Under the Influence of a Coronal Mass Ejection Sheath Region

“…The spacecraft potential provides the electron number density and the minimum resonance energy calculated is 1.2 MeV, which indicates that the EMIC wave packets observed in this event can resonantly interact with electrons ≥1.2 MeV. Consequently, EMIC waves may have scattered electrons to the loss cone (see e.g., Medeiros et al, 2019) during the high-energy electron flux dropout in the outer radiation belt under the influence of the ICME's sheath.…”

“…During the maximum magnetosphere compression (panel e and f), the modeled magnetopause boundary intercepts the 100 nT isocontour line, which would correspond to the drift paths of equatorially mirroring electrons near geosynchronous orbit. Thus, such electrons would be lost to the adjacent magnetosheath region upon interception with the magnetopause (see also Medeiros et al., 2019), therefore strongly suggesting that magnetopause shadowing significantly contributed to remove relativistic electrons from the outermost region of the outer Van Allen belt. We thus identify magnetopause shadowing as the first dynamical mechanism contributing to the high‐energy electron loss during the turbulent ICME's sheath region.…”

“…EMIC waves can have an important role in the loss of relativistic electrons to the upper atmosphere (see e.g., Medeiros et al., 2019; Sandanger et al., 2007; Thorne & Kennel, 1971; Usanova et al., 2014; X. J. Zhang et al., 2016). The ion composition is decisive to define how the wave‐particle interaction can occur.…”

“…EMIC waves appear mostly left-hand polarized, in the Pc1-2 ULF frequency range, that is, from 0.1 to 5 Hz, and the wave amplitude varies from 0.1 nT to 10s of nT (Halford et al, 2016). The local ion composition has been assumed (see e.g., Medeiros et al, 2019) here to define the wave-particle interaction effectiveness as well as wave amplitude, time duration, frequency band, and particle energy. The minimum resonant energy can be inferred by the Equation 1 (see e.g., Kang et al, 2016;Summers & Thorne, 2003):…”

“…EMIC waves appear mostly left‐hand polarized, in the Pc1‐2 ULF frequency range, that is, from 0.1 to 5 Hz, and the wave amplitude varies from 0.1 nT to 10s of nT (Halford et al., 2016). The local ion composition has been assumed (see e.g., Summers & Thorne, 2003 and Medeiros et al., 2019) here to define the wave‐particle interaction effectiveness as well as wave amplitude, time duration, frequency band, and particle energy. The minimum resonant energy can be inferred by the Equation (see e.g., Kang et al., 2016; Summers & Thorne, 2003): where, is the electron velocity parallel to the field line, m e is the electron rest mass, ω and k are the angular frequency and wave number of EMIC waves, respectively, Ω e is the electron gyrofrequency, c is the speed of light, n is the resonance harmonic number (assumed to be 1), and finally s is set to be −1 for the left‐hand mode.…”