Transition Region and Coronal Explorerand Soft X‐Ray Telescope Active Region Loop Observations: Comparisons with Static Solutions of the Hydrodynamic Equations

Abstract

Active region coronal loop observations with broadband X-ray instruments have often been found to be consistent with the predictions of static loop models. Recent observations in the EUV, however, have discovered a class of active region loops that are difficult to reconcile with static loop models. In this paper, we take a comprehensive look at how coronal loops compare with static models. We select 67 loops with a large range of apex temperatures and half-lengths observed with either the Transition Region and Coronal Explorer or the Soft X-Ray Telescope. We compare these observations to static loop models using both uniform and nonuniform heating. We find that only 2 of the 67 loops are fully consistent with static solutions with uniform heating and a filling factor of unity. We further find that long, cool (<3 MK) loops are as much as 2500 times "overdense," while short, hot (>3 MK) loops are as much as 63 times "underdense" when compared to the static solutions with uniform heating. We then consider the possibility that the disparity in the density could be due to steady, nonuniform heating along the loop and find that footpoint heating can increase densities only by a factor of 3 over density solutions with uniform heating while loop-top heating results in density solutions that are, at most, a factor of 2.5 smaller than the density solutions with uniform heating. Only 19 of the 67 loops in this data set could be fully consistent with hydrodynamic solutions with steady heating. Hence, we conclude that static loop models are poor representations of most active region loops.