标题： FOXSI-2 太阳微耀发 II：硬 X 射线成像光谱学和耀发能量学 显示英文标题

标题： FOXSI-2 Solar Microflares II: Hard X-ray Imaging Spectroscopy and Flare Energetics

摘要： 我们研究了在2014年12月11日第二次飞行的聚焦光学太阳X射线成像仪（FOXSI-2）探空火箭实验期间观测到的两个次A类太阳微耀斑的能量释放和能量传递特性。FOXSI是第一个利用聚焦光学器件在硬X射线（HXR）波段成像太阳的专门用于太阳的仪器，对4-20千电子伏的能量范围敏感。通过使用光学薄等温等离子体模型对这两个微耀斑进行光谱分析，我们发现了被加热至约10兆开尔文的等离子体以及降至约$10^{44}~$厘米$^{-3}$的辐射度证据。尽管未检测到FOXSI-2微耀斑中的非热发射，但对可能隐藏的非热分量参数空间的研究表明，非热电子中可能含有足够的能量来解释微耀斑1中的热能，这表明该耀斑与标准厚靶模型一致。通过采用优化的太阳设计和改进的硬X射线聚焦光学器件，FOXSI-2在10千电子伏处比核光谱望远镜阵列（NuSTAR）具有大约五倍更高的灵敏度，并允许首次对太阳硬X射线进行直接成像光谱学研究，其角分辨率适用于微耀斑尺度。利用这些改进的能力来研究小规模事件的演化，我们发现在次A类耀斑期间存在空间和时间上的复杂性。结合太阳动力学观测台（SDO/AIA）大气成像组件的同期观测研究表明，这些小规模微耀斑的演化更类似于大耀斑，而非预期的纳米耀斑单一能量爆发。 显示英文摘要

摘要： We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket experiment on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to the energy range 4-20 keV. Through spectral analysis of the two microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to temperatures of ~10 MK and emissions measures down to ~$10^{44}~$cm$^{-3}$. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array (NuSTAR) for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study the evolution of small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. These studies in combination with contemporanous observations by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) indicate that the evolution of these small microflares is more similar to that of large flares than to the single burst of energy expected for a nanoflare.