Investigating Loss of Relativistic Electrons Associated With EMIC Waves at Low L Values on 22 June 2015

Abstract

In this study, rapid loss of relativistic radiation belt electrons at low L * values (2.4-3.2) during a strong geomagnetic storm on 22 June 2015 is investigated along with five possible loss mechanisms. Both the particle and wave data are obtained from the Van Allen Probes. Duskside H + band electromagnetic ion cyclotron (EMIC) waves were observed during a rapid decrease of relativistic electrons with energy above 5.2 MeV occurring outside the plasmasphere during extreme magnetopause compression. Lower He + composition and enriched O + composition are found compared to typical values assumed in other studies of cyclotron resonant scattering of relativistic electrons by EMIC waves. Quantitative analysis demonstrates that even with the existence of He + band EMIC waves, it is the H + band EMIC waves that are likely to cause the depletion at small pitch angles and strong gradients in pitch angle distributions of relativistic electrons with energy above 5.2 MeV at low L values for this event. Very low frequency wave activity at other magnetic local time can be favorable for the loss of relativistic electrons at higher pitch angles. An illustrative calculation that combines the nominal pitch angle scattering rate due to whistler mode chorus at high pitch angles with the H + band EMIC wave loss rate at low pitch angles produces loss on time scale observed at L = 2.4-3.2. At high L values and lower energies, radial loss to the magnetopause is a viable explanation.

Modeling the Magnetopause Shadowing Loss During the June 2015 Dropout Event

“…Due to the limited coverage of equatorial pitch angles in the observed electron fluxes, we cannot sufficiently validate if the pitch angle dependence of the observed loss is consistent with EMIC wave scattering. In addition, Xiang et al (2017) and Qin et al (2019) both calculated the minimum resonant energy of the observed H + band EMIC waves and found it to be in the range of 4 to 5 MeV. Therefore, the observed loss of the storage ring at lower energies shown in Figure 3d could be due to scattering by other types of magnetospheric waves which also needs to be investigated in the future.…”

Statistical Dependence of EMIC Wave Scattering on Wave and Plasma Parameters

“…Future work will test the wave normal angle dependence with sufficient statistics. In addition to parameters that have been mentioned above, the minimum resonant energy and pitch angle diffusion coefficient are also sensitive to the cold ion composition (Bashir & Ilie, ; Jordanova et al, ; Meredith et al, ; Qin et al, ; Summers & Thorne, ). We did not include it in the current statistical study since direct measurement of low‐energy ions is affected by the spacecraft potential and cannot be detected down to lowest energies (Min et al, ).…”

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.…”