Cross Sections of Coronal Loop Flux Tubes. (arXiv:2007.15085v1 [astro-ph.SR])

Cross Sections of Coronal Loop Flux Tubes. (arXiv:2007.15085v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Klimchuk_J/0/1/0/all/0/1">James A. Klimchuk</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+DeForest_C/0/1/0/all/0/1">Craig E. DeForest</a>

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.

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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