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arXiv 1602.01112astro-ph.SRastro-ph.HEphysics.plasm-ph
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部分电离等离子体中重联驱动慢模激波的形成和演化

The formation and evolution of reconnection-driven slow-mode shocks in a partially ionised plasma

Andrew Hillier, Shinsuke Takasao, Naoki Nakamura

摘要:

慢模MHD激波在磁重联中的作用对于能量转换和输运是非常重要的,但在许多天体物理等离子体中,等离子体并不完全电离。在本文中,我们调查,使用数值模拟,质子-电子电荷-中性流体和中性氢流体之间的碰撞耦合的作用,在恒定的压力和密度的背景状态下发起的磁场的不连续性的一维黎曼问题。在磁流体极限下,该系统的特征在于两种波:快模式稀疏波,其驱动流向慢模式磁流体冲击。系统演化经历了初始、弱耦合、中间耦合和准稳态四个阶段。初始阶段的特征是一个超压的中性区域,该区域以爆炸波的特征膨胀。在后期阶段,系统趋向于自相似解,其中主要的漂移速度集中在冲击波阵面的薄区域。由于系统的性质,当与导致中性流体膨胀以形成激波前兆的纯流体动力学激波相比时,中性流体被激波超压。一旦激波阵面形成,它的厚度与电离分数的-1.2次方成正比,这是一个比从简单的标度参数天真地预期的指数更小的指数。一个有趣的结果是,激波阵面是一个连续的过渡的物理变量的亚音速上游的激波阵面(一个c-激波)的物理变量的急剧跳跃,然后由一个松弛的下游值的超音速上游速度(一个j-激波)。由穿过冲击波阵面的速度漂移引起的摩擦加热可以达到参考磁能的大约百分之二。

英文摘要:

The role of slow-mode MHD shocks in magnetic reconnection is one of great importance for energy conversion and transport, but in many astrophysical plasmas the plasma is not fully ionised. In this paper, we investigate, using numerical simulations, the role of collisional coupling between a proton-electron charge-neutral fluid and a neutral hydrogen fluid for the 1D Riemann problem initiated in a constant pressure and density background state by a discontinuity in the magnetic field. This system, in the MHD limit, is characterised by two waves: a fast-mode rarefaction wave that drives a flow towards a slow-mode MHD shock. The system evolves through four stage: initiation, weak coupling, intermediate coupling and a quasi steady state. The initial stages are characterised by an over-pressured neutral region that expands with characteristics of a blast wave. In the later stages, the system tends towards a self-similar solution where the main drift velocity is concentrated in the thin region of the shock front. Due to the nature of the system, the neutral fluid is overpressured by the shock when compared to a purely hydrodynamic shock which results in the neutral fluid expanding to form the shock precursor. The thickness of the shockfront once it has formed proportional to the ionisation fraction to the power -1.2, which is a smaller exponent than would be naively expected from simple scaling arguments. One interesting result is that the shock front is a continuous transition of the physical variables for sub-sonic velocity upstream of the shock front (a c-shock) to a sharp jump in the physical variables followed by a relaxation to the downstream values for supersonic upstream velocity (a j-shock). The frictional heating that results from the velocity drift across the shock front can amount to approximately two per cent of the reference magnetic energy.

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