THICKNESS VARIATIONS ALONG CORONAL LOOPS INFERRED FROM VECTOR MAGNETOGRAPH DATA

It has been noted for many years that images of active region coronal loops seen in soft X-rays or extreme ultraviolet emission suggest a pipelike appearance. Recently Klimchuk et al. have quantified this characteristic, finding for several loops observed by the Soft X-Ray Telescope on the Yohkoh spacecraft a thickness variation along their lengths of only 10%-20%. We demonstrate here that this observation is consistent with the characteristics of current-carrying field lines in a highly sheared active region. Vector magnetogram data on NOAA active region 5747, taken with the Stokes Polarimeter at Mees Solar Observatory on 1989 October 20, provided photospheric boundary conditions from which a force-free coronal magnetic field was computed. By tracing field lines, we show that magnetic loops which are highly sheared do not expand rapidly in height, as they would in a potential field. In addition, the expanding sections close to the footpoints of current-carrying twisted loops tend to be more vertical than in a potential field, so that when seen projected against the solar disk, the loops appear to terminate more abruptly. Consequently, current-carrying loops exhibit a near-uniform cross section with thickness variations of order 30% along their lengths.