Evolving Active Region Loops Observed with theTransition Region and Coronal Explorer. I. Observations

Abstract

Observations made with TRACE have detected a class of persistent active region loops that have flat 195/ 171 Å filter ratios. The intensity of these loops implies a density that is as much as 3 orders of magnitude larger than the densities of static solutions to the hydrodynamic equations. It has recently been suggested that these loops are bundles of impulsively heated strands that are cooling through the TRACE passbands. This scenario implies that the loops would appear in the hotter (Fe xv 284 Å or Fe xii 195 Å ) TRACE filter images before appearing in the cooler (Fe ix/x 171 Å ) TRACE filter images. In this paper, we test this hypothesis by examining the temporal evolution of five active region loops in multiple TRACE EUV filter images. We find that all the loops appear in the hotter filter images before appearing in cooler filter images. We then use the measured delay to estimate a cooling time and find that four of the five loops have lifetimes greater than the expected lifetime of a cooling loop. These results are consistent with the hypothesis that each apparent loop is a bundle of sequentially heated strands; other explanations will also be discussed. To facilitate comparisons between these loops and hydrodynamic simulations, we use a new technique to estimate the loop length and geometry.

An Investigation into the Variability of Heating in a Solar Active Region

“…The estimated lifetimes in all of the cases are significantly smaller than the observed ones. These results are consistent with the results of Winebarger et al (2003).…”

“…Our second choice for g(t) will be representative of impulsive heating, i.e., a triangular function of 2 duration , where 2 ¼ 1000 s is small compared to the lifetime of the loop. Because the observed lifetime of the loop in the TRACE 195 8 filter is much larger than the predicted lifetime of a cooling loop (see Table 3), we expect that the impulsive heating model will require the use of multiple strands to reproduce the lifetimes, similar to the previous work Winebarger et al 2003).…”

“…The fit was weighted by the uncertainties in the fluxes, which in turn were estimated from the statistical ( photons) and nonstatistical (instrumental) uncertainties in the original image. With better sampled light curves in the TRACE filters and, therefore, better constrained Gaussians, the measured delays between the 195 and 171 8 filters have smaller uncertainties and consistently indicate a delay in 171 8 ( positive difference) with respect to 195 8 (see also Winebarger et al 2003), except for one case (loop 3), which is simultaneous within the uncertainties. The delays measured between the cooler SXT filters and the TRACE 195 8 filter are between 20 and 90 minutes and are similar in scale to the delays measured by Winebarger & Warren (2005).…”

“…The first technique to determine the loop length was described completely by Winebarger et al (2003) and is briefly described here. We assume that the loop is a symmetric three-dimensional ellipse that is projected into the image plane.…”

“…Loops that are completely visible in the EUV, however, cannot be addressed by steady heating models. Even though these loops persist much longer than their characteristic cooling times, the filter ratio along the loops and large apex emission measures cannot be reproduced by steady heating models (Lenz et al 1999;Aschwanden et al 2001;Winebarger et al 2003). Warren et al (2002) found that an impulsive heating mechanism could produce the large densities needed to account for the emission, while a bundle of strands sequentially heated would reproduce the observed lifetimes.…”