Simulation of the energy distribution of relativistic electron precipitation caused by quasi‐linear interactions with EMIC waves

Abstract

[1]Previous studies on electromagnetic ion cyclotron (EMIC) waves as a possible cause of relativistic electron precipitation (REP) mainly focus on the time evolution of the trapped electron flux. However, directly measured by balloons and many satellites is the precipitating flux as well as its dependence on both time and energy. Therefore, to better understand whether pitch angle scattering by EMIC waves is an important radiation belt electron loss mechanism and whether quasi-linear theory is a sufficient theoretical treatment, we simulate the quasi-linear wave-particle interactions for a range of parameters and generate energy spectra, laying the foundation for modeling specific events that can be compared with balloon and spacecraft observations. We show that the REP energy spectrum has a peaked structure, with a lower cutoff at the minimum resonant energy. The peak moves with time toward higher energies and the spectrum flattens. The precipitating flux, on the other hand, first rapidly increases and then gradually decreases. We also show that increasing wave frequency can lead to the occurrence of a second peak. In both single- and double-peak cases, increasing wave frequency, cold plasma density or decreasing background magnetic field strength lowers the energies of the peak(s) and causes the precipitation to increase at low energies and decrease at high energies at the start of the precipitation.

Investigation of EMIC wave scattering as the cause for the BARREL 17 January 2013 relativistic electron precipitation event: A quantitative comparison of simulation with observations

“…Higher initial trapped flux, stronger waves, and broader interaction spatial coverage will cause the REP flux to increase and hence result in a higher X-ray count rate, and vice versa; weaker equatorial background magnetic field, higher wave frequency, and greater cold plasma density will not only cause the REP flux level to increase but also lower the energy of the peak in the REP energy spectrum, leading to a higher count rate as well as a softer X-ray energy spectrum, and vice versa [Li et al, 2013] (Shown in Figure 4c are two test simulations with cold plasma density increased and decreased by a factor of 2.5.). The presented approach of comparing the X-ray spectra should be further exploited in testing the role of EMIC waves in relativistic electron precipitation.…”

EMIC waves and plasmaspheric and plume density: CRRES results

“…It is thought that EMIC waves often are capable of resonating with ≥ 2 MeV electrons in the radiation belts [e.g., Silin et al , ; Chen et al , ]. However, it has been hypothesized that EMIC waves in plasmaspheric plumes are also capable of resonating with < 2 MeV radiation belt electrons due to the enhanced cold plasma density and relatively low B‐field found within the plumes leading to a reduction of phase speeds [e.g., Meredith et al , ; Summers et al , ; Ukhorskiy et al , ; Silin et al , ; Li et al , ]. The local density gradients possibly found within the plumes have been theorized to aid in larger EMIC wave growth due to forcing the wave normal with respect to the geomagnetic field potentially leading to more significant loss [ Summers and Thorne , ; Chen et al , ].…”

“…This region in the dusk sector is where the highest occurrence rates of EMIC waves are observed to overlap with the radiation belts [ Anderson et al , ; Fraser and Nguyen , ; Halford et al , ; Yuan et al , ; Usanova et al , ]. These waves are also potentially able to resonate with the lower energy, ≤1.5 MeV, radiation belt electrons [e.g., Meredith et al , ; Ukhorskiy et al , ; Silin et al , ; Li et al , , and A. Hendry et al, “Lower Energy cut‐off limits of EMIC‐driven energetic electron precipitation” submitted to Journal of Geophysical Research , 2015]. Other satellite studies which are able to sample at higher L ‐values [e.g., Usanova et al , ; Min et al , ] see a similar trend of an increase in EMIC occurrence as one moves to higher L ; however, as the outer radiation belt tends to extend out to and at times beyond L = 7 [e.g., Li and Temerin , ; Li et al , ; Baker and Kanekal , ; Hudson et al , ; Ni et al , ], this relatively Earthward population of EMIC waves are of great interest as they are the most likely to interact with radiation belt particles.…”

BARREL observations of an ICME‐shock impact with the magnetosphere and the resultant radiation belt electron loss

“…In particular there was a strong EMIC wave produced by this compression event (not shown). However, as EMIC waves have a minimum resonant energy for electrons at relativistic energies, this wave is not the source of the lower energy precipitation observed by BARREL 2L [ Li et al , , and references therein].…”