On the nature of the "moss" observed by TRACE

Moss " is the name given to low-lying (similar to 3000 km), hot (similar to 1 MK) solar coronal plasma that has been observed recently by the Transition Region and Coronal Explorer (TRACE). This paper investigates two hypotheses regarding the nature of the moss: (1) emission from small, million degree loops; (2) emission from the legs of 3-10 million degree loops. We update the coronal radiative loss curve, using the most recent results for coronal abundances, and use an analytical loop model to find that the first hypothesis requires a filling factor close to unity to reproduce the observed emission measure, while the second hypothesis results in a filling factor of about 0.1, in agreement with other independent multiwavelength analyses of moss. We find that the vertical extent and the height of the moss layer above the limb are also very well reproduced with the second hypothesis. We further show that the observed brightness of the moss scales linearly with the loop pressure and filling factor, independent of the loop length, and we derive a general expression for the conversion factor.