Hybrid simulations of the decay of reconnected structures downstream of the bow shock
Hybrid simulations of the decay of reconnected structures downstream of the bow shock
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DOI：
10.1063/5.0129084
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发表时间：
2023-01
影响因子：
2.2
通讯作者：
I. Gingell;S. Schwartz;H. Kucharek;C. Farrugia;L. J. Fryer;J. Plank;K. Trattner
中科院分区：
文献类型：
--
作者：
I. Gingell;S. Schwartz;H. Kucharek;C. Farrugia;L. J. Fryer;J. Plank;K. Trattner
磁层多尺度观测表明，磁重联发生在地球的弓形激波处，通常发生在激波过渡区等离子体不稳定性和湍流引起的薄电流片处。对激波转捩和下游磁鞘的观测研究表明，这些区域中电流片的数量可能并不强烈地依赖于激波马赫数MA或上游磁场与激波法线之间的夹角（θBn）。考虑到准平行和高马赫数激波往往具有更无序和非定常的结构，这个结果有些令人吃惊。为了研究如何冲击重联体现在不同的参数，我们进行了一系列的混合动力（流体电子，动力离子）的粒子在细胞模拟在一系列的马赫数和方向。鉴于混合模拟不能解决电子规模的电流片和重联，这些模拟隔离的离子-离子束不稳定性驱动的脚的冲击重联的离子规模的机制。我们发现，这种机制是强烈的约束，在所有模拟马赫数的准平行冲击。通过用闭合磁力线上等离子体所占面积来量化重联，发现在上游区和斜坡区，重联结构的数目和闭合场面积随MA的增加而增加，随θBn的增加而减小。下游的冲击，但是，我们发现一个类似的结果，观测调查：准平行冲击的子集内，封闭的字段区域（因此薄电流片）的衰减率并不强烈依赖于上游冲击参数。
Observations by Magnetospheric Multiscale have demonstrated that magnetic reconnection occurs at Earth's bow shock, typically at thin current sheets arising from plasma instabilities and turbulence in the shock transition region. Observational surveys of both the shock transition and the magnetosheath downstream suggest that the number of current sheets in these regions may not be strongly dependent on the shock Mach number MA or the angle between the upstream magnetic field and shock normal ( θBn). This result is somewhat surprising given that quasi-parallel and high Mach number shocks tend to have a more disordered and non-stationary structure. In order to investigate how shock reconnection manifests across different parameters, we perform a series of hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a range of Mach numbers and orientations. Given that hybrid simulations cannot resolve electron-scale current sheets and reconnection, these simulations isolate an ion-scale mechanism for shock reconnection driven by an ion–ion beam instability in the foot. We find that this mechanism is strongly constrained to quasi-parallel shocks across all simulated Mach numbers. By quantifying reconnection using the area occupied by plasma on closed magnetic field lines, we find the number of reconnecting structures and closed field area increase with MA and decrease with θBn in the upstream and ramp regions. Downstream of the shock, however, we find a similar result to observational surveys: within the subset of quasi-parallel shocks, the decay rate of the closed field area (and hence thin current sheets) is not strongly dependent on upstream shock parameters.