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 pi pelike appearance. Recently Klimchuk et al. have quantified this chara cteristic, finding for several loops observed by the Soft X-Ray Telesc ope on the Yohkoh spacecraft a thickness variation along their lengths of only 10%-20%. We demonstrate here that this observation is consist ent with the characteristics of current-carrying field lines in a high ly sheared active region. Vector magnetogram data on NOAA active regio n 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 li nes, we show that magnetic loops which are highly sheared do not expan d rapidly in height, as they would in a potential field. In addition, the expanding sections close to the footpoints of current-carrying twi sted loops tend to be more vertical than in a potential field, so that when seen projected against the solar disk, the loops appear to termi nate more abruptly. Consequently, current-carrying loops exhibit a nea r-uniform cross section with thickness variations of order 30% along t heir lengths.