Coronal Heat: Solar Loop Temperatures from TRACE Triple-Filter Data

Abstract

The Transition Region and Coronal Explorer (TRACE) has state-of-the-art spatial resolution and shows the most detailed images of coronal loops ever observed. The temperatures of these loops are primarily derived from the 171 to 195 Å filter ratio, with data from the third filter at 284 Å used by several authors to improve the precision of the derived temperatures. Most of these studies assume that the plasma is isothermal and model the loops primarily as uniform temperature structures with footpoint-dominated heating. However, these triple-filter data are insufficient to constrain the plasma temperature and cannot be used to determine the isothermality or otherwise of coronal loop structures. We show this explicitly by constructing differential emission measures with these same triple-filter data using a sophisticated Markov-chain Monte Carlo-based reconstruction algorithm. We find that these TRACE data cannot, in general, limit the temperature distribution for coronal loop plasma. In other words, many different temperature distributions (isothermal, broad, sloped, etc.) can reproduce the observed fluxes, and the TRACE coronal data alone cannot determine which of these distributions represents the actual coronal plasma.