Upstream Plasma Waves and Downstream Magnetic Reconnection at a Reforming Quasi-parallel Shock

Abstract

With the help of a two-dimensional particle-in-cell simulation model, we investigate the long-time evolution (near 100 Ω i 0 − 1 , where Ω i0 is the ion gyrofrequency in the upstream) of a quasi-parallel shock. Some of the upstream ions are reflected by the shock front, and their interactions with the incident ions excite low-frequency magnetosonic waves in the upstream. Detailed analyses have shown that the dominant wave mode is caused by the resonant ion–ion beam instability, and the wavelength can reach tens of ion inertial lengths. Although these plasma waves are directed toward the upstream in the upstream plasma frame, they are brought by the incident plasma flow toward the shock front, and their amplitude is enhanced during the approaching. The interaction of the upstream plasma waves with the shock leads to the cyclic reformation of the shock front, and the reformation period is slightly larger than 10 Ω i 0 − 1 . When crossing the shock front, these large-amplitude plasma waves are compressed and evolve into current sheets in the transition region of the shock. At last, magnetic reconnection occurs in these current sheets, accompanying the generation of magnetic islands. Simultaneously, there still exist plasma waves of another kind, with the wavelength of several ion inertial lengths in the ramp of the shock, which are excited by the nonresonant ion–ion beam instability. The current sheets in the transition region are distorted and broken into several segments when the plasma waves of this kind are transmitted into the downstream, where magnetic reconnection and the generated islands have a much smaller size. No obvious ion flow can be observed around some X-lines produced in the magnetic reconnection, and this implies that electron-only reconnection may occur.

Cross‐Scale Energy Transfer From Ion‐Scale to Electron‐Scale Waves in the Earth's Foreshock Region

“…Simulation and observation studies have suggested that the generation of LFWs is related to resonant interactions (Hellinger et al, 2007;Hull et al, 2020;Lu et al, 2024) or nonresonant interactions (H. Li et al, 2013;Wang et al, 2020). In our event, the obtained wave frequency f is ∼0.2 Hz, the wave phase velocity v ph is ∼300 km/s (mainly in the anti-sunward direction, -x in GSE), and the wave normal angle is ∼20°.…”

“…Low-frequency fast-magnetosonic waves (LFWs) usually have a wavelength larger than 1,000 km, comparable with the ion gyro-radii in the Earth's foreshock region (Hobrara et al, 2007;Wang et al, 2021). These waves are an important part of the macroscopic structure upstream of the bow shock and have been widely investigated over the last several decades (Gary et al, 1984;Hobrara et al, 2007;Lu et al, 2024;Tsurutani et al, 1983Tsurutani et al, , 2001Wang et al, 2020). They are right-hand polarized and typically have the frequencies less than ∼few Hertz, below the lower hybrid resonance frequency (Hobrara et al, 2007;Tsurutani et al, 1983Tsurutani et al, , 2001Wang et al, 2021).…”

“…They are right-hand polarized and typically have the frequencies less than ∼few Hertz, below the lower hybrid resonance frequency (Hobrara et al, 2007;Tsurutani et al, 1983Tsurutani et al, , 2001Wang et al, 2021). Previous studies have suggested that the generation of LFWs is related to resonant interactions (Gary et al, 1984;Hellinger et al, 2007;Hull et al, 2020;Lu et al, 2024) or nonresonant interactions (Gary et al, 1984;H. Li et al, 2013;Wang et al, 2020Wang et al, , 2021.…”