ENERGY RELEASE AND TRANSFER IN SOLAR FLARES: SIMULATIONS OF THREE-DIMENSIONAL RECONNECTION
ENERGY RELEASE AND TRANSFER IN SOLAR FLARES: SIMULATIONS OF THREE-DIMENSIONAL RECONNECTION
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DOI：
10.1088/0004-637x/695/2/1151
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发表时间：
2009-04
期刊：
影响因子：
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通讯作者：
J. Birn;L. Fletcher;M. Hesse;T. Neukirch
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文献类型：
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作者：
J. Birn;L. Fletcher;M. Hesse;T. Neukirch
利用三维磁流体力学模拟研究了标准双带耀斑图像的三维扩展中的能量释放和传递。在这种情况下，重联在双极磁场上方的薄电流片（建议在离开的日冕物质抛射下形成）中启动。对比了两种情况：初始无力电流片（低β）和有限压电流片（高β），其中β表示气体（等离子体）和磁压之间的比率。重联后的能量转换过程包括入射波印廷通量转化为上行和下行波印廷通量、焓通量和体动能通量。在低情况下，向外的坡印亭通量是主要的贡献，而向外的焓通量在高情况下占主导地位。总体动能通量在向下方向上的贡献很小。在低β情况下，坡因廷通量下降的优势与Fletcher & Hudson最近提出的太阳耀斑能量输运的厚目标电子束模型的另一种选择是一致的，而焓通量可能是另一种输运机制。对于重连场结构的可信特征参数，我们获得了能量释放时间尺度和能量输出率，与从耀斑脉冲阶段的观测推断的结果比较有利。即使在最初无力的情况下，背景β也非常小，但焓通量和加热也很明显，这主要是由绝热压缩而不是欧姆耗散造成的。能量转换机制最容易理解为一个两步过程（尽管这两个步骤可能基本上同时发生）：第一步是由洛伦兹力在类似于佩切克重联模型中缓慢冲击的层中加速等离子体，涉及将磁能转换为大块动能。然而，由于压力梯度力近似地或部分力平衡地反对洛伦兹力，加速的等离子体变慢并被压缩，从而大块动能转化为热量，要么局部沉积，要么通过焓通量转移并稍后沉积。这种机制在下流区最为相关，下流区受力平衡的支配更强；在重联位置上方的流出处，它不太重要，在那里更多的能量以快速散装流的形式保留。
Using three-dimensional magnetohydrodynamic simulations we investigate energy release and transfer in a three-dimensional extension of the standard two-ribbon flare picture. In this scenario, reconnection is initiated in a thin current sheet (suggested to form below a departing coronal mass ejection) above a bipolar magnetic field. Two cases are contrasted: an initially force-free current sheet (low beta) and a finite-pressure current sheet (high beta), where beta represents the ratio between gas (plasma) and magnetic pressure. The energy conversion process from reconnection consists of incoming Poynting flux turned into up- and downgoing Poynting flux, enthalpy flux, and bulk kinetic energy flux. In the low-beta case, the outgoing Poynting flux is the dominant contribution, whereas the outgoing enthalpy flux dominates in the high-beta case. The bulk kinetic energy flux is only a minor contribution in the downward direction. The dominance of the downgoing Poynting flux in the low-beta case is consistent with an alternative to the thick target electron beam model for solar flare energy transport, suggested recently by Fletcher & Hudson, whereas the enthalpy flux may act as an alternative transport mechanism. For plausible characteristic parameters of the reconnecting field configuration, we obtain energy release timescales and energy output rates that compare favorably with those inferred from observations for the impulsive phase of flares. Significant enthalpy flux and heating are found even in the initially force-free case with very small background beta, resulting mostly from adiabatic compression rather than Ohmic dissipation. The energy conversion mechanism is most easily understood as a two-step process (although the two steps may occur essentially simultaneously): the first step is the acceleration of the plasma by Lorentz forces in layers akin to the slow shocks in the Petschek reconnection model, involving the conversion of magnetic energy to bulk kinetic energy. However, due to pressure gradient forces that oppose the Lorentz forces in approximate, or partial force balance, the accelerated plasma becomes slowed down and compressed, whereby the bulk kinetic energy is converted to heat, either locally deposited or transported away by enthalpy flux and deposited later. This mechanism is most relevant in the downflow region, which is more strongly governed by force balance; it is less important in the outflow above the reconnection site, where more energy remains in the form of fast bulk flow.