Coronal rain corresponds to cool and dense clumps in the corona accreting towards the solar surface; it is often observed above solar active regions. These clumps are generally thought to be produced by a thermal instability in the corona and their lifetime is limited by the time they take to reach the chromosphere. Although the rain usually fragments into smaller clumps while falling down, their specific spatial and temporal scales remain unclear. In addition, the observational signatures of the impact of the rain with the chromosphere have not been clarified yet. In this study, we investigate the time evolution of the velocity and intensity of coronal rain above a sunspot by analyzing coronal images obtained by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) as well as the slit-jaw images (SJIs) and spectral data taken by the Interface Region Imaging Spectrograph (IRIS) satellite. We identify dark and bright threads moving towards the umbra in AIA images and in SJIs, respectively, and co-spatial chromospheric intensity enhancements and redshifts in three IRIS spectral lines, Mg ii k 2796 Å, Si iv 1394 Å, and C ii 1336 Å. The intensity enhancements and coronal rain redshifts occur almost concurrently in all the three lines, which clearly demonstrates the causal relationship with coronal rain. Furthermore, we detect bursty intensity variation with a time scale shorter than 1 minute in Mg ii k, Si iv , and C ii , indicating that a length scale of rain clumps is about 2.7 Mm if we multiply the typical time scale of the busty intensity variation at 30 sec by the rain velocity at 90 km s − 1 $90~\mbox{km}\,\mbox{s}^{-1}$ . Such rapid enhancements in the IRIS lines are excited within a time lag of 5.6 sec limited by the temporal resolution. These temporal and spatial scales may reflect the physical processes responsible for the rain morphology, and are suggestive of instabilities such as the Kelvin–Helmholtz instability.

中文翻译：

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紫外成像和光谱观测揭示的日冕雨的时空尺度

日冕雨对应于日冕中向太阳表面吸积的凉爽而密集的团块；它经常在太阳活动区上方观察到。这些团块通常被认为是由日冕中的热不稳定性产生的，它们的寿命受到它们到达色球所需的时间的限制。虽然雨在落下时通常会碎裂成更小的团块，但它们的具体时空尺度仍不清楚。此外，雨对色球层影响的观测特征尚未阐明。在这项研究中，我们通过分析太阳动力学天文台 (SDO) 上的大气成像组件 (AIA) 获得的日冕图像以及狭缝图像 (SJI) 和光谱来研究太阳黑子上方日冕雨的速度和强度的时间演变。界面区域成像光谱仪 (IRIS) 卫星获取的数据。我们分别在 AIA 图像和 SJI 中识别出向本影移动的暗线和亮线，以及三个 IRIS 谱线 Mg ii k 2796 Å、Si iv 1394 Å 和 C ii 1336 Å 中的同空间色球强度增强和红移. 强度增强和日冕雨红移几乎同时发生在所有三条线，这清楚地表明了与日冕雨的因果关系。此外，我们在 Mg ii k、Si iv 和 C ii 中检测到时间尺度短于 1 分钟的突发强度变化，表明如果我们乘以丰满强度变化的典型时间尺度，雨团的长度尺度约为 2.7 Mm 30 秒由 90 km s − 1 $90~\mbox{km}\,\mbox{s}^{-1}$ 的雨速计算。IRIS 线中的这种快速增强在 5.6 秒的时间延迟内受到时间分辨率的限制。这些时空尺度可能反映了造成降雨形态的物理过程，并暗示了不稳定性，例如开尔文-亥姆霍兹不稳定性。IRIS 线中的这种快速增强在 5.6 秒的时间延迟内受到时间分辨率的限制。这些时空尺度可能反映了造成降雨形态的物理过程，并暗示了不稳定性，例如开尔文-亥姆霍兹不稳定性。IRIS 线中的这种快速增强在 5.6 秒的时间延迟内受到时间分辨率的限制。这些时空尺度可能反映了造成降雨形态的物理过程，并暗示了不稳定性，例如开尔文-亥姆霍兹不稳定性。