Why Are Coronal Loops So Symmetric?

J. A. KLIMCHUK; M. C. LÓPEZ FUENTES; P. DÉMOULIN

Durham, New Hampshire, Estados Unidos

Congreso; 37th Meeting of the AAS Solar Physics Division; 2006

American Astronomical Society

Coronal loops are observed to be very symmetric in the sense that the two legs have a comparable thickness. Magnetic flux tubes in magnetic field extrapolation models are typically much less symmetric. We have quantified these differences using 171 A images from TRACE and magnetograms from MDI/SOHO. For a sample of 20 different loops, we found the linear force-free field that best matches the observed loop. We then measured the plane-of-the-sky widths of the loops and corresponding flux tubes and computed footpoint-to-footpoint expansion factors (i.e., asymmetry ratios). The mean expansion factor of the flux tubes is 2.62, whereas the mean expansion factor of the loops is only 1.35. Note that these expansion factors are different from the footpoint-to-midpoint expansion factors that we have presented previously.Evidence suggests that the coronal magnetic field is comprised elemental flux strands that are tangled by turbulent convection. These strands are so small that many tens of them are contained within a single TRACE loop. We suggest that this fine structure is a critical missing ingredient of the extrapolation models and that a combination of footpoint shuffling and coronal reconnection can explain the observed loop symmetry. This has important implications for coronal heating.