Cross Sections of Coronal Loop Flux Tubes

James A Klimchuk,Craig E Deforest

doi:10.3847/1538-4357/abab09

Abstract

Abstract 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 nonnegligible 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 subresolution 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.

Highlights

It has been recognized for many years that coronal loops---thin curved intensity features in coronal images---have widths that are approximately uniform along their length (Klimchuk et al 1992)
The columns in the table list, from left to right, the loop number, probability that I and w are random, most probable α, 90% confidence interval for α, average of the background-subtracted peak intensity in normalized instrument counts, average of the loop-to-background intensity ratio, and length of the measured loop section in pixels
We find that a sizable majority of the loops either have a clear positive correlation, α > 0, or are consistent with no correlation

Summary

Introduction

It has been recognized for many years that coronal loops---thin curved intensity features in coronal images---have widths that are approximately uniform along their length (Klimchuk et al 1992). Malanushenko & Schrijver (2013) suggested that non-circular cross sections might reconcile the apparent inconsistency between the observed uniform loop widths and the expected expansion of the field. Klimchuk (2000) pointed out that the approximate uniformity of width in observed coronal loops suggests that the cross sections are roughly circular.

Results

Conclusion