论文标题
我们可以信任MHD跳跃条件的无碰撞冲击吗？
Can we trust MHD jump conditions for collisionless shocks?
论文作者
论文摘要
当应用用于计算冲击的密度跳跃时，标准的磁性流动力（MHD）形式主义假定，1）所有上游材料都会下游，以及它所携带的动量和能量，以及2）压力是各向同性的。在无碰撞冲击中，冲击加速粒子在前面来回走动可能是第一个假设无效的。此外，外部磁场可以维持稳定的压力各向异性，使第二个假设无效。因此，尚不清楚无碰撞冲击的密度跳跃是否能够实现MHD跳跃。 在这里，我们试图澄清这个问题。对68篇涉及无碰撞冲击粒子模拟的文章进行了文献综述。我们分析了触发MHD密度跳跃的因素，并量化了它们对$δ_{rh} $的影响，这是与Rankine-Hugoniot跳跃相对偏离的。对于小出发，我们提出$δ_{rh} = + \ \ \ \ \ \ \ \ \ \ \ \ \}（10^{ - 1-3.7κ}）t^κ-σ\ Mathcal {o}（1）$，其中$ t $是模拟的时间范围，$σ$ $σ$磁力化参数和$κ$ corder and conders of Constant of Constant an Constant andity andity andity andity andity。第一项源于能量泄漏到加速粒子中。第二项源自该场触发的下游各向异性（假设上游）。这种关系允许评估无碰撞冲击能够达到RH密度跳跃的程度。 在强场极限和平行冲击中，该场引起的偏离在有限的，负值的情况下饱和。对于垂直的冲击，在小$σ$的小额和高$σ$上的偏离为零，因此我们在这里找到了一个出发窗口。必须针对完整的3D模拟检查获得的结果。
When applied to compute the density jump of a shock, the standard magnetohydrodynamic (MHD) formalism assumes, 1) that all the upstream material passes downstream, together with the momentum and energy it carries, and 2) that pressures are isotropic. In a collisionless shock, shock accelerated particles going back and forth around the front can invalid the first assumption. In addition, an external magnetic field can sustain stable pressure anisotropies, invaliding the second assumption. It is therefore unclear whether the density jump of a collisionless shock fulfils the MHD jump or not. Here we try to clarify this issue. A literature review is conducted on 68 articles dealing with Particle-In-Cell simulations of collisionless shocks. We analyze the factors triggering departure from the MHD density jump and quantify their influence on $Δ_{RH}$, the relative departure from the Rankine-Hugoniot jump. For small departures we propose $Δ_{RH} = + \mathcal{O}(10^{-1-3.7κ})t^κ- σ\mathcal{O}(1)$ where $t$ is the timescale of the simulation, $σ$ the magnetization parameter and $κ$ a constant of order unity. The first term stems from the energy leakage into accelerated particle. The second term stems from the downstream anisotropy triggered by the field (assuming an isotropic upstream). This relation allows to assess to which extent a collisionless shock fulfils the RH density jump. In the strong field limit and for parallel shocks, the departure caused by the field saturates at a finite, negative, value. For perpendicular shocks, the departure goes to zero at small and high $σ$'s so that we find here a departure window. The results obtained have to be checked against full 3D simulations.
