A nanoflare distribution generated by repeated relaxations triggered by kink instability
A nanoflare distribution generated by repeated relaxations triggered by kink instability
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DOI：
10.1051/0004-6361/201014067
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发表时间：
2010-10-01
影响因子：
6.5
通讯作者：
Van der Linden, R. A. M.
中科院分区：
文献类型：
--
作者：
Bareford, M. R.;Browning, P. K.;Van der Linden, R. A. M.
语境。人们认为，大量的纳米耀斑可能是导致日冕温度达到数百万度的原因。目前的观测技术缺乏证实纳米耀斑假说的分辨能力。另一种方法是构建磁流体动力学冠状环模型，该模型能够预测纳耀斑能量分布。本文介绍了冠状环模型生成的初始结果，只要它变得不稳定到理想的 MHD 扭结模式，该模型就会耀斑。该模型的一个特点是，它可以预测各种尺寸的加热事件，具体取决于遇到线性扭结模式的不稳定阈值的位置。目的是计算事件能量的分布并研究是否可以从单个参数预测扭结不稳定性。方法。该环被表示为直线连接的圆柱体。由随机光球运动引起的扭曲由两个参数捕获，代表环路特定区域的电流密度与场强的比率。不稳定开始被映射为二维参数空间中的闭合边界。环路磁能的耗散在不稳定的非线性阶段开始，这是由于电流片重新连接而产生的。展开后，环路演变成能量最低的状态，根据弛豫理论，电流与场的比率在整个环路中是恒定的，并且螺旋性是守恒的。结果。环路状态的径向磁扭曲分布沿不稳定阈值存在显着变化。这些结果表明，任何简单的扭曲衍生特性都无法达到临界值来预测不稳定性。应用该模型使得环经历反复的不稳定发作，然后释放能量松弛。因此，对所产生的纳米耀斑的能量分布进行了整理。本文还介绍了所有不稳定阈值点的计算弛豫状态和能量释放。结论。最终的能量分布具有遵循不同幂律的两个纳米耀斑群体。高能量群体的幂律指数对于日冕加热来说绰绰有余。
Context. It is thought likely that vast numbers of nanoflares are responsible for the corona having a temperature of millions of degrees. Current observational technologies lack the resolving power to confirm the nanoflare hypothesis. An alternative approach is to construct a magnetohydrodynamic coronal loop model that has the ability to predict nanoflare energy distributions.Aims. This paper presents the initial results generated by a coronal loop model that flares whenever it becomes unstable to an ideal MHD kink mode. A feature of the model is that it predicts heating events with a range of sizes, depending on where the instability threshold for linear kink modes is encountered. The aims are to calculate the distribution of event energies and to investigate whether kink instability can be predicted from a single parameter.Methods. The loop is represented as a straight line-tied cylinder. The twisting caused by random photospheric motions is captured by two parameters, representing the ratio of current density to field strength for specific regions of the loop. Instability onset is mapped as a closed boundary in the 2D parameter space. Dissipation of the loop's magnetic energy begins during the nonlinear stage of the instability, which develops as a consequence of current sheet reconnection. After flaring, the loop evolves to the state of lowest energy where, in accordance with relaxation theory, the ratio of current to field is constant throughout the loop and helicity is conserved.Results. There exists substantial variation in the radial magnetic twist profiles for the loop states along the instability threshold. These results suggest that instability cannot be predicted by any simple twist-derived property reaching a critical value. The model is applied such that the loop undergoes repeated episodes of instability followed by energy-releasing relaxation. Hence, an energy distribution of the nanoflares produced is collated. This paper also presents the calculated relaxation states and energy releases for all instability threshold points.Conclusions. The final energy distribution features two nanoflare populations that follow different power laws. The power law index for the higher energy population is more than sufficient for coronal heating.