Reformation of rippled quasi‐parallel shocks: 2‐D hybrid simulations

Abstract

One‐dimensional (1‐D) hybrid simulations have demonstrated that a quasi‐parallel shock is nonstationary and undergoes a reformation process. Recently, two‐dimensional (2‐D) hybrid simulations have revealed that ripples along the shock front is an inherent property of a quasi‐parallel shock. In this paper, we investigate reformation process of a rippled quasi‐parallel shock with a 2‐D hybrid simulation model. The simulation results show that at a rippled shock, incident particles behave differently and just can be partially reflected at some specific locations along the rippled shock front, and the reflected particles will form an ion beam that moves back to the upstream along the magnetic field. Then, the beam locally interacts with upstream waves, and the waves are enhanced and finally steepen into a new shock front. As the upstream incident plasma moves to the shock front, the new shock front will approach and merge with the old shock front. Such a process occurs only before these locations along the shock front, and after the merging of the new shock front and old shock front is finished, a relatively plane shock front is formed. Subsequently, a new rippled shock front is again generated due to its interaction with the upstream waves, and it will repeat the previous process. In this pattern, the shock reforms itself quasiperiodically, and at the same time, ripples can shift along the shock front. The simulations present a more complete view of reformation for quasi‐parallel shocks.

Impacts of Magnetosheath High‐Speed Jets on the Magnetosphere and Ionosphere Measured by Optical Imaging and Satellite Observations

“…Hietala et al () suggested that HSJs are probably generated due to the ripples of the quasi‐parallel bow shock, and a quantitative study shows that 97% of the observed HSJs can be produced by bow shock ripples (Hietala & Plaschke, ). The result of 2‐D hybrid simulation is also consistent with this proposed mechanism (Karimabadi et al, ; Hao et al, , ).…”

Statistical Study of Magnetosheath Jet‐Driven Bow Waves

“…Magnetosheath jets are more likely to occur downstream of the quasi-parallel rather than the quasi-perpendicular bow shock (e.g., Plaschke et al, 2013;Vuorinen et al, 2019). One possible reason is that the surface of the quasi-parallel bow shock is rippled (e.g., Gingell et al, 2017;Hao et al, 2017;Karimabadi et al, 2014). If the solar wind crosses the bow shock where the surface is locally tilted, the downstream flow will be less thermalized and decelerated, thus forming a magnetosheath jet that is colder and faster than the ambient magnetosheath flow (e.g., Hietala et al, 2009;.…”

Electron Acceleration by Magnetosheath Jet‐Driven Bow Waves

“…Magnetosheath jets are typically~1 R E in size (e.g., Plaschke et al, 2016) and occur nine times more often downstream of the quasi-parallel bow shock (the angle between upstream magnetic field and the bow shock normal θ Bn < 45°) than downstream of the quasi-perpendicular bow shock (θ Bn > 45°) (e.g., Vuorinen et al, 2019). The widely accepted explanation for this is that the quasi-parallel bow shock is very unstable with many ripples on its surface (e.g., Gingell et al, 2017;Hao et al, 2017;Karimabadi et al, 2014). When the solar wind crosses such a locally tilted surface, it is less thermalized and less decelerated than in the surrounding areas, resulting in a localized downstream flow that is colder and faster than the ambient magnetosheath (e.g., Hietala et al, 2009;.…”