Plasma diagnostics of transition region "moss" using SOHO/CDS and TRACE

Recent observations of solar active regions with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying EUV emiss ion, called the "moss," appearing as a bright dynamic pattern with dark inc lusions. The moss has been interpreted as the upper transition region by Be rger and coworkers. In this study we use SOHO Coronal Diagnostic Spectromet er and TRACE observations of Active Region 8227 on 1998 May 30 to determine the physical parameters of the moss material. We establish that the plasma responsible for the moss emission has a temperature range of (0.6-1.5) x 1 0(6) K and is associated with hot loops (T> 2 x 10(6) K). Moss plasma has a n electron density of (2-5) x 10(9) cm(-3) at a temperature of 1.3 x 106 K, giving a pressure of 0.7-1.7 dynes cm(-2) (a few times higher than in coro nal loops observed in the TRACE Fe IX/X lambda 171 passband). The volume fi lling factor of the moss plasma is of order 0.1, and the path along which t he emission originates is of order 1000 km long.