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arXiv 1701.02196astro-ph.SR
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极紫外线引起的太阳耀斑中的非高斯速度分布:离子加速的一种可能诊断

Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration

Natasha L. S. Jeffrey, Lyndsay Fletcher, Nicolas Labrosse

摘要:

在太阳耀斑中,释放的大部分磁能迅速转化为非热粒子和大块等离子体运动的动能。这将可能导致非平衡粒子分布和湍流等离子体条件。我们通过分析由日出号上的EUV成像光谱仪(EIS)观测到的一次大耀斑(SOL 2014 -03- 29 T17:44)的高温极紫外发射线的轮廓来研究这一点。我们发现,在许多位置的线轮廓是非高斯的,一致的卡帕分布的发射离子的属性,在空间和时间上的变化。在耀斑的脚点,接近非热电子发射硬X射线的位置,Fe XVI 262.976埃线在3 MK的卡帕指数取3-5的值。在日冕中,接近低能HXR源,Fe XXIII 263.760埃线在15 MK显示kappa值通常为4-7。观察到的kappa参数的趋势表明,我们最有可能检测到的离子群体的属性,而不是任何仪器的影响。我们计算出,如果在< 0.1 s的时间尺度上局部加速,则可能存在非热离子群。然而,观察到的净红移线也意味着存在等离子体的下行流,这可能会导致大量的湍流,在较冷的地区增加非高斯性。这两种解释对太阳耀斑粒子加速理论都有重要意义。

英文摘要:

In a solar flare, a large fraction of the magnetic energy released is converted rapidly to the kinetic energy of non-thermal particles and bulk plasma motion. This will likely result in non-equilibrium particle distributions and turbulent plasma conditions. We investigate this by analysing the profiles of high-temperature extreme ultraviolet emission lines from a major flare (SOL2014-03-29T17:44) observed by the EUV Imaging Spectrometer (EIS) on Hinode. We find that in many locations the line profiles are non-Gaussian, consistent with a kappa-distribution of emitting ions with properties that vary in space and time. At the flare footpoints, close to sites of hard X-ray emission from non-thermal electrons, the kappa-index for the Fe XVI 262.976 angstrom line at 3 MK takes values of 3-5. In the corona, close to a low-energy HXR source, the Fe XXIII 263.760 angstrom line at 15 MK shows kappa values of typically 4-7. The observed trends in the kappa parameter show that we are most likely detecting the properties of the ion population rather than any instrumental effects. We calculate that a non-thermal ion population could exist if locally accelerated on timescales < 0.1 s. However, observations of net redshifts in the lines also imply the presence of plasma downflows which could lead to bulk turbulence, with increased non-Gaussianity in cooler regions. Both interpretations have important implications for theories of solar flare particle acceleration.

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