Cooling Active Region Loops Observed With SXT and TRACE

Abstract

An Impulsive Heating Multiple Strand (IHMS) Model is able to reproduce the observational characteristics of EUV (~ 1 MK) active region loops. This model implies that some of the loops must reach temperatures where X-ray filters are sensitive (> 2.5 MK) before they cool to EUV temperatures. Hence, some bright EUV loops must be preceded by bright X-ray loops. Previous analysis of X-ray and EUV active region observations, however, have concluded that EUV loops are not preceded by X-ray loops. In this paper, we examine two active regions observed in both X-ray and EUV filters and analyze the evolution of five loops over several hours. These loops first appear bright in the X-ray images and later appear bright in the EUV images. The delay between the appearance of the loops in the X-ray and EUV filters is as little as 1 hour and as much as 3 hours. All five loops appear as single ``monolithic'' structures in the X-ray images, but are resolved into many smaller structures in the (higher resolution) EUV images. The positions of the loops appear to shift during cooling implying that the magnetic field is changing as the loops evolve. There is no correlation between the brightness of the loop in the X-ray and EUV filters meaning a bright X-ray loop does not necessarily cool to a bright EUV loop and vice versa. The progression of the loops from X-ray images to EUV images and the observed substructure is qualitatively consistent with the IMHS model.