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 Transiti on Region and Coronal Explorer (TRACE). This paper investigates two hypothe ses regarding the nature of the moss: (1) emission from small, million degr ee loops; (2) emission from the legs of 3-10 million degree loops. We updat e the coronal radiative loss curve, using the most recent results for coron al abundances, and use an analytical loop model to find that the first hypo thesis requires a filling factor close to unity to reproduce the observed e mission 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 ab ove the limb are also very well reproduced with the second hypothesis. We f urther show that the observed brightness of the moss scales linearly with t he loop pressure and filling factor, independent of the loop length, and we derive a general expression for the conversion factor.