Abstract
We conduct test particle simulations to examine the acceleration mechanism of relativistic electrons through interaction with multi-subpacket chorus waves. As the analysis of recent observations reveals, amplitude of a rising tone element of chorus wave consists of many short wave packets. We call this single rising tone chorus element with the collective structure of multiple short wave packets as a multi-subpacket chorus wave. In this simulation, we develop the wave model with rapidly fluctuating amplitude and phase discontinuities across each subpacket, in order to examine how these features of multi-subpacket chorus wave influence the nonlinear trapping processes in efficient acceleration of relativistic electrons such as relativistic turning acceleration (RTA) and ultra-relativistic acceleration (URA). To conduct comprehensive examinations, we test more than nine million particles with various initial conditions covering the energy range from 100 to 6 MeV, and the equatorial pitch angles from 10° to 89°. The test particles interact with a single rising tone element of multi-subpacket chorus wave set up with the maximum amplitude of about 2 nT and the frequency rise from about 1.3 kHz to 3.8 kHz over 0.25 s. Relativistic electrons are accelerated by about 160 keV under preferable conditions. The energy increase verifies the high efficiency of acceleration by the wave–particle interactions, based on the fact that it is achieved by a short time interaction less than 1 s with a single element of chorus wave. By analyzing the detailed behavior of the accelerated electrons, we find successive trapping of the resonant electrons resulting in the efficient accelerations from the consecutive multiple subpackets of a chorus wave element.
Highlights
Prompt recovery of highly energetic (MeV) electron populations in Earth’s outer radiation belt, the outermost of the twin torus-shaped rings of high-energy plasma surrounding Earth, has been observed during geomagnetic storms (e.g., Foster et al 2017)
Summary and conclusion By recent observations from the Van Allen Probes (Foster et al 2017; Omura et al 2019), it is seen that amplitude of a single rising tone element of chorus wave consists of a large number of short wave packets, defined as multisubpacket chorus wave
Since the trapping region, where resonant electrons can be efficiently accelerated, depends on the wave amplitude, the multi-subpacket structure causes a rapid variation of the trapping region and affects the processes of trapping energetic electrons
Summary
Prompt recovery of highly energetic (MeV) electron populations in Earth’s outer radiation belt, the outermost of the twin torus-shaped rings of high-energy plasma surrounding Earth, has been observed during geomagnetic storms (e.g., Foster et al 2017). Hiraga and Omura Earth, Planets and Space (2020) 72:21 chorus wave at the resonance velocity VR , some of them fall into the wave potential, which is formed by Lorentz force of electron velocities perpendicular to Earth’s dipole magnetic field v⊥ and the wave magnetic field. These electrons continue to remain in resonance being phasetrapped in the potential, resulting in efficient acceleration such as RTA and URA. We conduct simulations of wave–particle interaction including the multi-subpacket wave structure and the phase discontinuities, and examine the efficiency of the electron acceleration processes (RTA and URA) under these conditions. We summarize our simulation results and conclude this study in “Summary and conclusion” section
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