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arXiv 2007.15085astro-ph.SR
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冕环磁通管的截面

Cross Sections of Coronal Loop Flux Tubes

James A. Klimchuk, Craig E. DeForest

备注：
27 pages, 9 figures, accepted by Astrophysical Journal
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摘要：

日冕环揭示了日冕磁场和日冕加热的本质的重要信息。相应的通量管截面的形状以及它如何随位置变化是特别重要的特性。它们是场的扩展和加热的跨场空间分布的直接指示。我们已经研究了20环使用高空间分辨率的观测从第一次飞行的Hi-C火箭实验，测量的强度和宽度作为一个函数的位置沿环轴。我们发现，强度和宽度往往要么是不相关的，或有一个直接的依赖，使他们增加或减少一起。这意味着通量管截面在假设管具有不可忽略的扭曲并且等离子体发射率沿着磁场近似均匀的情况下近似为圆形。形状不需要是正圆，并且发射率在截面内不需要是均匀的，但是发射的亚分辨率补丁必须在具有单位阶纵横比的包络内准均匀地分布。这就对通量管随高度而膨胀的说法提出了质疑，但主要是在视线方向上，因此相应的（相对明显的）环似乎具有大致均匀的宽度，这是一个长期存在的难题。这也让人对大多数环对应于简单的翘曲片的想法产生了怀疑，尽管我们保留了更复杂的流形结构的可能性。

英文摘要：

Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have non-negligible twist and that the plasma emissivity is approximately uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but sub-resolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle. It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures.

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