ELECTRON ENERGY PARTITION IN THE ABOVE-THE-LOOPTOP SOLAR HARD X-RAY SOURCES
ELECTRON ENERGY PARTITION IN THE ABOVE-THE-LOOPTOP SOLAR HARD X-RAY SOURCES
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DOI：
10.1088/0004-637x/799/2/129
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发表时间：
2014-12
期刊：
影响因子：
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通讯作者：
M. Oka;S. Krucker;H. Hudson;P. Saint-Hilaire
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文献类型：
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作者：
M. Oka;S. Krucker;H. Hudson;P. Saint-Hilaire
太阳耀斑产生能量高达数十兆电子伏的非热电子。为了了解高能电子的起源，人们对日冕硬 X 射线 (HXR) 源，特别是环上源进行了广泛研究。然而，目前仍不清楚环路顶部源内的热电子和非热电子之间的能量如何分配。在这里，我们表明，与传统的光谱模型相比，kappa 分布可以更好地表征在四种不同情况下观察到的环上 HXR (≳15 keV)。广泛使用的传统模型（即组合热力加幂律分布）也可以拟合数据，但由于非物理急剧的低能量截止Ec，它返回不合理的参数值。在两个案例中，极紫外数据可从 SDO/AIA 获得，并且 kappa 分布仍然与差分发射测量的分析一致。根据kappa分布模型，我们发现2012年7月19日的耀斑显示出最大的电子能量非热部分，约为50%，表明能量均分。考虑到细胞内粒子模拟的结果以及所研究的四种情况的密度估计，我们提出了一种场景，其中电子加速主要通过无碰撞磁重联实现，但环顶上方源中的电子能量分配取决于源密度。在环顶上方的低密度区域（109 cm−3 的几倍），增强的非热尾部可以保留，并创建显着的 HXR 源，而在较高密度（>1010 cm−3）中，非热尾部被库仑碰撞抑制或热化。
Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray (HXR) sources, in particular above-the-looptop sources, have been studied extensively. However, it still remains unclear how energies are partitioned between thermal and non-thermal electrons within the above-the-looptop source. Here we show that the kappa distribution, when compared to conventional spectral models, can better characterize the above-the-looptop HXRs (≳15 keV) observed in four different cases. The widely used conventional model (i.e., the combined thermal plus power-law distribution) can also fit the data, but it returns unreasonable parameter values due to a non-physical sharp lower-energy cutoff Ec. In two cases, extreme-ultraviolet data were available from SDO/AIA and the kappa distribution was still consistent with the analysis of differential emission measure. Based on the kappa distribution model, we found that the 2012 July 19 flare showed the largest non-thermal fraction of electron energies about 50%, suggesting equipartition of energies. Considering the results of particle-in-cell simulations, as well as density estimates of the four cases studied, we propose a scenario in which electron acceleration is achieved primarily by collisionless magnetic reconnection, but the electron energy partition in the above-the-looptop source depends on the source density. In low-density above-the-looptop regions (few times 109 cm−3), the enhanced non-thermal tail can remain and a prominent HXR source is created, whereas in higher-densities (>1010 cm−3), the non-thermal tail is suppressed or thermalized by Coulomb collisions.