A closer look at a coronal loop rooted in a sunspot umbra
A closer look at a coronal loop rooted in a sunspot umbra
复制标题
DOI：
10.1051/0004-6361/201527340
复制
发表时间：
2015-12
期刊：
影响因子：
--
通讯作者：
L. P. Chitta;H. Peter;P. Young;P. Young
中科院分区：
文献类型：
--
作者：
L. P. Chitta;H. Peter;P. Young;P. Young
太阳日冕中的极紫外线（EUV）和X射线环连接着磁活动增强的区域，但它们通常不会植根于太阳黑子的黑暗阴影中，因为那里发现的强磁场抑制了对流。这意味着，只要没有光桥或本影点，进入高层大气的磁能坡印廷通量在本影内就不重要。在这里，我们报告了一个罕见的观测日冕环植根于黑暗的本影的太阳黑子没有任何痕迹的光桥或本影点。我们使用的狭缝颚图像和光谱数据从IRIS和集中的线配置文件的O IV和Si IV，显示持久的强红移成分的循环植根于本影。利用O-IV的比值，我们可以估计密度，从而研究质量通量。通过AIA的EUV通道提供这些观测的日冕背景和温度诊断。日冕环，嵌入在较冷的下降流，主机超音速下降流。超过100 km/s的速度在O IV和Si IV的过渡区线中处于相同的数量级，甚至在色球Mg II线中也可以看到类似的速度。在距足点1”$以内的投影距离处，我们看到激波过渡到约15 km s$^{-1}$的较小下降流速度，这与静止等温激波的质量守恒是一致的。我们没有看到能量输入到环中的直接证据，因为环植根于本影的黑暗均匀部分，附近没有光桥或本影点。因此，人们可能会得出结论，我们看到的是从循环另一端的足点驱动的虹吸流。然而，对于最终结果，需要在另一个足点处具有类似质量的数据，但这太远而不能被IRIS视场覆盖。
Extreme UV (EUV) and X-ray loops in the solar corona connect regions of enhanced magnetic activity, but they are not usually rooted in the dark umbrae of sunspots because the strong magnetic field found there suppresses convection. This means that the Poynting flux of magnetic energy into the upper atmosphere is not significant within the umbra as long as there are no light bridges or umbral dots. Here we report a rare observation of a coronal loop rooted in the dark umbra of a sunspot without any traces of light bridges or umbral dots. We used the slit-jaw images and spectroscopic data from IRIS and concentrate on the line profiles of O IV and Si IV that show persistent strong redshifted components in the loop rooted in the umbra. Using the ratios of O IV, we can estimate the density and thus investigate the mass flux. The coronal context and temperature diagnostics of these observations is provided through the EUV channels of AIA. The coronal loop, embedded within cooler downflows, hosts supersonic downflows. The speed of more than 100 km s$^{-1}$ is on the same order of magnitude in the transition region lines of O IV and Si IV, and is even seen at comparable speed in the chromospheric Mg II lines. At a projected distance of within $1"$ of the footpoint, we see a shock transition to smaller downflow speeds of about 15 km s$^{-1}$ being consistent with mass conservation across a stationary isothermal shock. We see no direct evidence for energy input into the loop because the loop is rooted in the dark uniform part of the umbra with no light bridges or umbral dots near by. Thus one might conclude that we are seeing a siphon flow driven from the footpoint at the other end of the loop. However, for a final result data of similar quality at the other footpoint are needed, but this is too far away to be covered by the IRIS field of view.