Simulations of Electron Flux Oscillations as Observed by MagEIS in Response to Broadband ULF Waves

Abstract

Coherent electron flux oscillations of hundreds of keV are often observed by the Van Allen Probes in the magnetosphere during quiet times in association with ultralow frequency (ULF) waves. They are observed in the form of periodic flux fluctuations, with a drift frequency that is energy dependent, but are not associated with drift echoes following storm‐ or substorm‐related energetic particle injections. Instead, they are associated with the resonant interaction of electrons with ULF waves and are an indication of ongoing electron radial diffusion. To investigate details of such flux oscillations, particle‐tracing simulations are conducted under the effect of realistic, broadband ULF electric and consistent magnetic fluctuations. Virtual detectors are simulated along spacecraft orbits and the results are compared to measurements. Through a parametric study, it is found that the width of electron energy channels is a critical parameter affecting the observed amplitude of flux oscillations, with narrower energy channel widths enabling the observation of higher‐amplitude flux oscillations; this potentially explains why such features were not observed regularly before the Van Allen Probes era, as previous spacecraft generally had lower energy resolution, which only enabled the observation of large‐amplitude drift echoes following a storm or substorm. Results are confirmed using the Magnetic Electron Ion Spectrometer (MagEIS) ultrahigh energy resolution data. Energy width effects are quantified through a parametric simulation study that matches flux oscillation observations during a period that is characterized by extremely quiet conditions, where the Van Allen Probes observed flux oscillations over multiple days.

Relation Between Shock‐Related Impulse and Subsequent ULF Wave in the Earth's Magnetosphere

“…Our result indicates that the excitation of shock‐related ULF waves is more likely determined by internal local condition in the magnetosphere. This study may be helpful for understanding the effect of shock on Earth's magnetosphere and quantification of radiation belt electron, as ULF waves can greatly affect magnetospheric electron population (e.g., Hao et al, 2014; Ren et al, 2017; Sarris et al, 2017, 2020; Zhou et al, 2016; Zong et al, 2009). Further observations and simulation on how shock‐induced ULF wave distributes in realistic magnetosphere are in need to be carried out in future work.…”

The Relativistic Electron-Proton Telescope (REPT) Investigation: Design, Operational Properties, and Science Highlights

“…Stochastic radial transport, more commonly referred to as radial diffusion, results from the interaction of particles with a spectrum of waves with frequencies commensurate with the drift frequency of the particles (e.g. Elkington et al 2003), although repetitive advective processes such as substorms may lead to the diffusive radial transport of particles over longenough timescales (Chen et al 1992;Sarris et al 2017Sarris et al , 2020. Radial diffusion will tend to move particles in aggregate in the direction opposite the prevailing gradients of the local phase space density.…”

On the Association Between Electron Flux Oscillations and Local Phase Space Density Gradients

“…Instead, they can be linked to the resonant interaction of electrons with ULF waves and, as discussed in Sarris et al. (2020), they could be used as an indicator of electron radial diffusion. It was demonstrated that critical in the observations of flux oscillations is the width of electron detector energy channels, with narrower energy channels enabling the detection of flux oscillations of smaller amplitude.…”