Impact of Background Magnetic Field for EMIC Wave‐Driven Electron Precipitation

Abstract

Wave-particle interaction between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves is a highly debated loss process contributing to the dynamics of Earth's radiation belts. Theoretical studies show that EMIC waves can result in strong loss of relativistic electrons in the radiation belts , https://doi.org/10.1029. However, many of these studies have not been validated by observations. Li et al. (2014, https://doi.org/10.1002/2014GL062273) modeled the relativistic electron precipitation observed by Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) in a single-event case study based on a quasi-linear diffusion model and observations by Van Allen Probes and GOES 13. We expand upon that study to investigate the localization of the precipitation region and the effectiveness of EMIC waves as an electron loss mechanism.The model results of BARREL 1I observations on 17 January 2013 show that as the BARREL balloon drifts in local time to regions that map to lower equatorial magnetic field strength, the flux of precipitating electrons increases and peaks at lower energy. The hypothesis that the energy of the precipitating electrons is controlled by background magnetic field strength is further tested by considering observations from balloon campaigns conducted from 2000 to 2014, including BARREL. Consistent with theory for wave-particle interaction between relativistic electrons and EMIC waves, we find observationally that stronger equatorial magnetic field strength generally correlates with more energetic electron precipitation and further conclude that magnetic field strength can drive the localization and distribution of precipitating electrons.A number of studies have investigated the statistical distribution of EMIC waves as observed by a single spacecraft. Anderson et al. (1992) used AMPTE CCE, which covered an L range from 3.5 to 9 at all local times, and found a peak wave occurrence (10%-20%) in the dayside magnetosphere at L > 7, with a break in WOODGER ET AL.8518

Statistical Dependence of EMIC Wave Scattering on Wave and Plasma Parameters

“…This is consistent with the result in Qin et al (). The coincidence occurrence rate is especially low in the year 2014 compared to other periods as seen in Table , which demonstrates the correlation between EMIC wave driven REP events with trapped flux levels. The percentage of EMIC waves coincident with REP increases with respect to background electron density, ratio of plasma frequency to local gyrofrequency, and EMIC wave power, as seen in Figures and . This is consistent with the theoretical analysis (Summers & Thorne, ). The coincidence occurrence rate decreases as the magnetic field increases between 120 and 270 nT (Figure ), consistent with a previous study (Woodger et al, ). The low backround plasma density might counteract the effect of the magnetic field in determining the coincidence occurrence rate with magnetic field below 120 nT.…”

Pitch Angle Scattering of Sub‐MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves

“…While both EMIC waves and precipitation events are more likely to occur on the duskside of the Earth, there is a significant difference in the distribution (as discussed by, e.g., Smith et al, ). While the peak occurrence of EMIC waves (especially for He band waves) seems to be in the afternoon sector (MLT = 12–18) (Anderson et al, ; Halford et al, ; Min et al, ; Saikin et al, ), the distribution of relativistic electron precipitation events seems to be shifted into the premidnight sector (MLT = 18–24; Carson et al, ; Comess et al, ; Woodger et al, ). The Carson et al () database with simultaneous proton precipitation (on which the Hendry et al, , , results were based) finds the distribution of events extending past midnight to about MLT = 2, and the occurrence at MLT = 12 is very low compared to the occurrence of EMIC waves.…”

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.…”