双带耀斑磁重联过程的数值研究

NUMERICAL STUDY OF MAGNETIC RECONNECTION PROCESSES IN TWO-RIBBON FLARES

本文用多步隐格式求解包含电阻的磁流体力学方程组,对双带耀斑的主相作数值模拟,清晰地展示了中性片区由撕裂模线性重联向准稳态重联的过渡以及后随耀斑环的产生和等离子体团的喷发过程.对于在能量方程中计及和忽略焦耳加热两种情况,分别作了计算.结果表明,计及焦耳加热时,电流片中等离子体的温度显著增加(是初始温度的2—3倍),但等离子体的运动速度却变化不大.两种情况的计算结果均表明:等离子体的运动速度低于声速,因此不会形成快激波.计及焦耳加热的计算结果显示了两个新的特征:其一是中性片高密度等离子体的受热膨胀,增大了电流片的有效厚度,它使重联速率降低,并逐渐趋于饱和,其二是同时形成上升和沉降等离子体团,后者与耀斑环碰撞,并合并于后随耀斑环内.

This paper is aimed at a numerical simulation of the main phase of two-ribbon flares by solving viscous MHD equations in terms of a multistep implicit scheme. The numerical results clearly show the transition from linear reconnection of tearing modes to quasisteady re-connection in the neutral sheet, the formation of post-flare loops, and the ejection of plas-moids.Two cases are considered respectively: one includes and the other does not include Joule heating in the energy equation. It is shown that including Joule heating leads to a substantial increase of temperature (with a factor of 2-3), but a negligible variation of velocity for the plasma within the neutral sheet. The plasma motions are always subsonic for both cases and therefore, no fast shocks are formed.The numerical results of including heating show two new features. One is a reduction and a tendency to saturation of the reconnection rate which is caused by the expansion of the high-density plasma in the neutral sheet, leading to an increase in the effective thickness of the neutral sheet. The other is the formation of ascending and descending plasmoids, and the coalesces of the latter with the post-flare loops.