Spectral properties and associated plasma energization by magnetosonic waves in the Earth's magnetosphere: Particle‐in‐cell simulations

Abstract

In this paper, we perform a 1‐D particle‐in‐cell (PIC) simulation model consisting of three species, cold electrons, cold ions, and energetic ion ring, to investigate spectral structures of magnetosonic waves excited by ring distribution protons in the Earth's magnetosphere, and dynamics of charged particles during the excitation of magnetosonic waves. As the wave normal angle decreases, the spectral range of excited magnetosonic waves becomes broader with upper frequency limit extending beyond the lower hybrid resonant frequency, and the discrete spectra tends to merge into a continuous one. This dependence on wave normal angle is consistent with the linear theory. The effects of magnetosonic waves on the background cold plasma populations also vary with wave normal angle. For exactly perpendicular magnetosonic waves (parallel wave number k|| = 0), there is no energization in the parallel direction for both background cold protons and electrons due to the negligible fluctuating electric field component in the parallel direction. In contrast, the perpendicular energization of background plasmas is rather significant, where cold protons follow unmagnetized motion while cold electrons follow drift motion due to wave electric fields. For magnetosonic waves with a finite k||, there exists a nonnegligible parallel fluctuating electric field, leading to a significant and rapid energization in the parallel direction for cold electrons. These cold electrons can also be efficiently energized in the perpendicular direction due to the interaction with the magnetosonic wave fields in the perpendicular direction. However, cold protons can be only heated in the perpendicular direction, which is likely caused by the higher‐order resonances with magnetosonic waves. The potential impacts of magnetosonic waves on the energization of the background cold plasmas in the Earth's inner magnetosphere are also discussed in this paper.

Two‐Dimensional Particle‐in‐Cell Simulation of Magnetosonic Wave Excitation in a Dipole Magnetic Field

“…As the waves are strongest near Ω p 0 t = 40, electron perpendicular kinetic energy reaches the maximum, while as the waves decay later, the perpendicular energy vanishes because of weakening E × B drift. The energization characteristics found in our 2‐D simulation of the cool protons and electrons are consistent with previous 1‐D simulation in a homogeneous plasma (Sun et al, ), which offers detailed diagnosis and detailed explanation. One noteworthy point in our 2‐D simulation is that cool electron and proton energization is more efficient near the source region than outside the source region (Figures e and f), because of the greater wave intensity inside.…”

Particle‐in‐Cell Simulations of the Fast Magnetosonic Mode in a Dipole Magnetic Field: 1‐D Along the Radial Direction

“…Corresponding to the wave energy, Figure shows the azimuthal component of proton kinetic energy normalized to the initial value. The azimuthal component is chosen to diagnose the perpendicular heating because the sloshing motion of background protons—a linear response to the pumping wave—occurs predominantly in the direction of spatial variation (i.e., radial direction for the present case) (e.g., Sun et al, , Figure 9). Corresponding to the wave absorption, the heating appears to occur near the seventh harmonic at in the warmer plasma.…”

In situ observations of magnetosonic waves modulated by background plasma density

“…Since MS waves were discovered several decades ago, a large number of studies, including theoretical works and particle‐in‐cell simulations, satellite observations, and statistical surveys, have been carried out to investigate the generation of MS waves. The results demonstrate that the background plasma and source particles are the two key factors to the wave generation [ Kennel , ; Chen et al ., ; Sun et al ., , , ]. Especially, a recent study, using detailed satellite data, has revealed that MS waves can be locally excited by protons with a ring distribution in velocity space [ Balikhin et al ., ].…”