The influence of rotational discontinuities on the formation of reconnected structures at collisionless shocks - hybrid simulations

Abstract

Recent simulations and in-situ observations have shown that magnetic reconnection is an active dissipation mechanism in the transition region of collisionless shocks. The generation mechanisms and upstream conditions enabling reconnection have been studied numerically. However, these numerical studies have been limited to the case of a steady, uniform upstream. The effect upstream discontinuities have on shock reconnection remains poorly understood. Here, we use local hybrid (fluid electron, particle ion) simulations with time-varying upstream conditions to study the influence upstream rotational discontinuities (RDs) have on the formation of reconnected magnetic structures in the shock transition region. Our results show that bursts of reconnection can occur when RDs interact with the shock. This effect is much more significant at initially quasi-parallel shocks than quasi-perpendicular shocks, as the interaction between the RDs and the foreshock (only present in the quasi-parallel case) can lead to the generation of foreshock bubbles in which we observe an enhanced reconnection occurrence. In addition, we find that the RDs with large magnetic shear are prone to reconnect upon reaching the shock, resulting in the generation of large magnetic islands. Our findings illustrate that upstream discontinuities can significantly increase the amount of reconnected magnetic structures at the bow shock, suggesting that reconnection might be a particularly important dissipation mechanism during periods of dynamic upstream conditions.

Enhanced Ion Acceleration Due To High‐Shear Tangential Discontinuities Upstream of Quasi‐Perpendicular Shocks

“…Source 2 first appears downstream of the shock after the shock‐TD collision, and moves downstream (in the shock frame) with the shocked plasma, continuing to energize ions for $$\tilde 10{\omega }_{\mathit{ci0}}^{-1}$$. This corresponds to the energetic ions being produced near the TD in Figure 1d, and is likely a result of local processes such as magnetic reconnection of the TD, which can be triggered when the TD is compressed by the shock (Hamrin et al., 2019; Lin, 1997; Steinvall & Gingell, 2024a). Source 3, which only appears in the Harris case, propagates faster into the downstream than the plasma flow, and generates a small number of energetic ions.…”

“…The local 2.5D hybrid code used here is the same as in Steinvall and Gingell (2024a). The code builds on the fusion of the full PIC code EPOCH (Arber et al., 2015) with the current advance method and cyclic leapfrog (CAM‐CL) algorithm (Matthews, 1994), as presented by Gingell et al.…”

“…The simulation data and MATLAB codes used to produce the figures in this article are publicly available at Steinvall and Gingell (2024b).…”

The Influence of Rotational Discontinuities on the Formation of Reconnected Structures at Collisionless Shocks—Hybrid Simulations

“…The simulation data and MATLAB codes used to produce Figures 2-5 are publicly available at (Steinvall & Gingell, 2023). MMS data are publicly available at https://lasp.colorado.edu/mms/sdc/public/.…”