Coronal loops really do have constant cross sections!

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

Honolulu, Hawaii

Congreso; 210th Meeting of the American Astronomical Society; 2007

American Astronomical Society

The observation that coronal loops do not expand systematically with height has been one of the more intriguing puzzles in solar physics. Simple force-free magnetic field models based on extrapolated magnetograms predict a much larger expansion than is observed. It has been suggested that the cross section uniformity is an artifact of inadequate spatial resolution, complex background emission, or both. For example, loops that are everywhere thinner than the instrument point spread function (PSF) would be seen to have a nearly constant thickness even if they actually expand. We have argued previously that actual loops are wide enough to rule out this possibility. Our present work also rules out the background emission as a possible explanation. We have simulated TRACE observations in the following manner. We constructed synthetic loops with both uniform and expanding cross sections, convolved them with the PSF, and placed them on actual TRACE images. We then measured the widths of the loops using the same technique used in our earlier studies of real observations. We find that expanding loops can be readily distinguished from loops with a constant cross section. Thus, the enigma remains! We tentatively suggest that constant cross sections are a consequence of the complex internal structure of loops (e.g., loops as bundles of tangled elemental strands). We are confident that this can explain the observed symmetry of loops, but whether it can also explain the lack of systematic expansion with height is not at all clear.