Interactions between magnetosonic waves and ring current protons: Gyroaveraged test particle simulations

Abstract

Gyroaveraged test particle simulations are implemented to quantitatively investigate interactions between linearly polarized magnetosonic waves (i.e., equatorial noises) and ring current protons inside and outside the plasmasphere at L = 4.5. For magnetosonic waves at the frequency of 33.3 Hz (fw/fcp = 6.4 at the magnetic equator, for L = 4.5), it is found that wave‐particle interactions at the resonance order corresponding to the lowest resonant proton energy (i.e., N = 6) are dominant. The interactions at other resonance orders make much less contribution. Near the equatorial loss cone at L = 4.5, magnetosonic waves produce strongest proton pitch angle diffusion at ~ 20 keV inside the plasmasphere and at ~ 100 keV outside the plasmasphere, respectively, reaching a rate above 10−6 s−1. The corresponding energy diffusion dominates over pitch angle diffusion at high pitch angles; therefore, magnetosonic waves are likely to accelerate protons at a few keV inside the plasmasphere and at ~ 10 keV outside the plasmasphere. Due to the emission equatorial confinement, the effect of transit time scattering also occurs for interactions of magnetosonic waves with ring current protons and tends to be increasingly important outside the plasmasphere, which is consistent with previous studies on interactions of magnetosonic waves with radiation belt electrons.

Resonant Scattering of Radiation Belt Electrons by Off‐Equatorial Magnetosonic Waves

“…Recent studies further stated that scattering by MS waves can provide an efficient mechanism for energetic electron diffusion in the slot region and outer radiation belt and primarily account for the formation of electron butterfly or top‐hat distributions in the inner magnetosphere (Li, Ni, et al, ; Li, Bortnik, et al, ; Ni et al, ; Xiao, Yang, et al, ). In addition, Fu et al () found that MS waves can potentially accelerate ring current protons at a few keV inside the plasmasphere and at ~10 keV outside the plasmasphere.…”

The Rapid Responses of Magnetosonic Waves to the Compression and Expansion of Earth's Magnetosphere

“…Although fast magnetosonic (MS) waves can also affect the ring current ions and the radiation belt electrons (Fu et al, ; Horne et al, ; Li, Ni, et al, ; Li et al, ; Ma et al, ; Xiao et al, ), their responses to the magnetospheric compression and expansion are not clear so far. MS waves are almost linearly polarized and propagate nearly perpendicular to the ambient magnetic field (Chen & Thorne, ; Russell et al, ; Santolík et al, ).…”

Ring current proton scattering by low-frequency magnetosonic waves

“…On the one hand, numerous studies have found that these emissions are able to produce electron butterfly distributions via Landau resonance and bounce resonance energization (Horne et al, ; Shprits, ; Bortnik and Thorne, ; Li JX et al, ; Li LY et al, ; Ni BB et al, , ; Tao X and Li X, ; Xiao FL et al, ). On the other hand, MS waves are also found capable of interacting with protons through cyclotron resonance (Xiao FL et al, ; Fu S et al, ). However, the interaction between MS waves and protons has drawn much less attention.…”

“…Based on quasi‐linear theory, Xiao FL et al () proposed that MS wave‐induced proton precipitation is a potential mechanism accounting for the proton aurora. Recently, the test particle simulations by Fu S et al () indicated that MS waves have the ability to accelerate ring current protons both inside and outside the plasmasphere.…”

“…Some observations and simulations have demonstrated that MS wave‐induced pitch angle and energy diffusion are able to contribute significantly to the dynamics of ring current protons (e.g. Xiao FL et al, ; Fu S et al, ). Previous studies have concentrated mainly on effects of high‐frequency MS waves ( f >20 Hz) on ring current protons; effects of low‐frequency ( f <20 Hz) MS waves on ring current ions have so far been lacking, despite the fact that low‐frequency MS waves have commonly been observed in the magnetosphere, especially in the high‐density plasmasphere ( Perraut et al, ; Posch et al, ).…”