Introduction
Relativistic electron flux dynamics in the Earth’s inner magnetosphere are largely controlled by electron scattering into the atmosphere via resonant interactions with whistler-mode and electromagnetic ion cyclotron (EMIC) waves . Near the loss-cone, electron scattering rates for EMIC waves are much larger than for whistler-mode waves \cite<e.g.,>{Glauert&Horne05,Summers07:rates,Ni15} and, thus, EMIC wave-driven electron precipitation is often considered as the main (spatially localized) loss mechanism for relativistic electrons with an energy exceeding the minimum energy for cyclotron resonance with such waves, \(E_{\min}\sim 0.5-1\)MeV . Series of numerical simulations of the outer radiation belt dynamics and data/model comparison have demonstrated that EMIC waves may quickly scatter relativistic electrons and contribute to the depletion of their flux in the outer radiation belt.
Spacecrafts and Dataset
- At \(\sim\) 01:15 UT ERG observed strong electron injections likely supporting whistler-mode wave generation (the onset of whistler-mode chorus waves coincides with this injection)
- At 01:30-02:30 UT GOES16&17 observed strong ion injections that arrived at ELFIN’s MLT (\(\sim16.5\)) around 02:30-03:00 UT (based on ion azimuthal drift estimates) and should have driven EMIC wave generation
- At 02:40-06:00 UT ELFIN observed continuous precipitation of relativistic electrons at MLT\(\sim 16\); NOAA/POES observations suggest precipitations are located right at the inner edge of the ion plasma sheet; to support such precipitations by EMIC waves, whistler-mode waves recorded by ERG (at MLT \(\sim 20\)) should continuously scatter relativistic electrons from higher equatorial pitch-angles into the pitch-angle range resonating with EMIC waves
- At 07:10-07:30 UT ERG and GOES16&17 observed a strong electron injection: dispersionless on ERG (MLT \(\sim 20\)) and dispersive on GOES 17 (MLT \(\sim4\)); This injection appears to restore electron fluxes and to largely compensate losses from EMIC wave-driven scattering, at least at \(E\leq1.5\) MeV
Discussion and Conclusions
In this paper, we have investigated a particular event on 17 April 2021 characterized by series of strong electron and ion injections from the plasma sheet, significant electron precipitation by EMIC and whistler-mode chorus waves, and electron acceleration by chorus waves. During this event, GOES, Van Allen Probes, ERG (ARASE) and MMS spacecraft have measured waves and trapped particle fluxes at high altitude near the magnetic equator, while ELFIN and POES spacecraft have recorded trapped and precipitating particle fluxes at low altitude, providing sufficient data to enable a thorough analysis of the involved physical phenomena.
Although ELFIN and POES measurements have shown that EMIC and chorus waves did efficiently precipitate \(\sim0.1-1.5\) MeV electrons in the outer radiation belt during this event, trapped electron fluxes actually increased at nearly all energies. Combining theoretical estimates of electron quasi-linear pitch-angle and energy diffusion by chorus and EMIC waves with statistics of their wave power distribution, we have shown that long-lasting electron losses driven by EMIC waves may not deplete \(\sim0.1-1.5\) MeV electron fluxes in the outer radiation belt over the long run (\(>8\) hours) in the case of a sufficiently negative derivative \(\partial f/\partial E<0\) of the electron PSD \(f(E)\), because this negative PSD gradient can lead to a strong transport of low-energy injected electrons toward higher energy through efficient chorus wave-driven electron acceleration, more than compensating relativistic electron losses due to EMIC and chorus wave-driven precipitation into the atmosphere – although a brief initial net loss at high energy can cause an early decrease of \(\gtrsim 1\) MeV electron flux . In addition, electron injections from the plasma sheet, measured near \(L\approx 7\) by GOES, may have been sufficiently strong after 7 UT to compensate electron losses due to wave-driven electron precipitation below \(\sim1.5\) MeV at \(L=5-6.5\), leading together with chorus wave-driven acceleration to a net increase of relativistic elec tron fluxes. This case study therefore underlines the fact that strong EMIC and chorus wave-driven electron losses do not necessarily correspond to a simultaneous decrease of trapped electron fluxes. Both local electron energy PSD gradients and radial PSD gradients and injections can balance such wave-driven losses. Therefore, they should be included in global codes to accurately calculate the dynamical evolution of trapped fluxes.