重复微耀斑活动区的NuSTAR观测
NuSTAR observations of a repeatedly microflaring active region
摘要：
我们研究了2018年9月9日和10日在X射线中观察到的AR 12721的10个微耀斑的空间，时间和光谱特性，使用核光谱望远镜ARray（NuSTAR）和太阳动力学天文台的大气成像组件以及日震和磁成像仪（SDO/AIA和HMI）。我们发现GOES次A类等效微耀斑能量为10 $^{26}$ -10 $^{28}$尔格，温度高达10 MK，与一致的静止或热活动区核心等离子体温度为3-4 MK。其中一个微耀斑（SOL 2018 -09- 09 T10：33）的等效GOES等级为A0.1，在其脉冲阶段（非热功率$\sim$ 7 $\times$ 10 $^{24}$ erg s $^{-1}$）具有非热HXR发射，使其成为具有加速电子直接证据的最微弱的X射线微耀斑之一。在10个微耀斑中的4个中，我们发现X射线时间分布与EUV中更微弱和更瞬态的源相匹配，突出了仅对达到高于活动区核心温度的最热材料（$>$ 5 MK）敏感的观测的需要。在8个微耀斑的足点处也观察到相应的光球磁通量抵消/出现的证据。
英文摘要：
We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager (SDO/AIA and HMI). We find GOES sub-A class equivalent microflare energies of 10$^{26}$-10$^{28}$ erg reaching temperatures up to 10 MK with consistent quiescent or hot active region core plasma temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal HXR emission during its impulsive phase (of non-thermal power $\sim$7$\times$10$^{24}$ erg s$^{-1}$) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the EUV, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the active region core ($>$5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of 8 microflares is also observed.
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