Hnat Bogdan, Chapman Sandra, Watkins Nicholas
Physics Department, Centre for Fusion Space and Astrophysics, University of Warwick, Coventry, UK.
Department of Mathematics and Statistics, University of Tromsø, Tromsø, Norway.
Sci Rep. 2023 Oct 31;13(1):18665. doi: 10.1038/s41598-023-45650-x.
In near-collisionless plasmas, which are ubiquitous in astrophysics, entropy production relies on fully-nonlinear processes such as turbulence and reconnection, which lead to particle acceleration. Mechanisms for turbulent reconnection include multiple magnetic flux ropes interacting to generate thin current sheets which undergo reconnection, leading to mixing and magnetic merging and growth of coherent structures, unstable reconnection current layers that fragment and turbulent reconnection outflows. All of these processes act across, and encompass, multiple reconnection sites. We use Magnetospheric Multi Scale four-point satellite observations to characterize the magnetic field line topology within a single reconnection current layer. We examine magnetopause reconnection where the spacecraft encounter the Electron Diffusion Region (EDR). We find fluctuating magnetic field with topology identical to that found for dynamically evolving vortices in hydrodynamic turbulence. The turbulence is supported by an electron-magnetohydrodynamic (EMHD) flow in which the magnetic field is effectively frozen into the electron fluid. Accelerated electrons are found in the EDR edge where we identify a departure from this turbulent topology, towards two-dimensional sheet-like structures. This is consistent with a scenario in which sub-ion scale turbulence can suppress electron acceleration within the EDR which would otherwise be possible in the electric field at the X-line.
在天体物理学中普遍存在的近无碰撞等离子体中，熵的产生依赖于湍流和重联等完全非线性过程，这些过程会导致粒子加速。湍流重联机制包括多个相互作用的磁通绳产生薄电流片，电流片发生重联，导致混合、磁合并以及相干结构的增长，不稳定的重联电流层破碎以及湍流重联外流。所有这些过程都跨越并包含多个重联位点。我们利用磁层多尺度四点卫星观测来表征单个重联电流层内的磁力线拓扑结构。我们研究磁层顶重联，即航天器遇到电子扩散区（EDR）的情况。我们发现波动磁场的拓扑结构与在流体动力学湍流中动态演化的涡旋所发现的拓扑结构相同。这种湍流由电子磁流体动力学（EMHD）流支持，其中磁场有效地冻结在电子流体中。在EDR边缘发现了加速电子，在那里我们确定从这种湍流拓扑结构向二维片状结构转变。这与一种情况相符，即亚离子尺度湍流可以抑制EDR内的电子加速，否则在X线处的电场中电子加速是可能的。
