Plasma Diagnostics of Transition Region “Moss” using [ITAL]SOHO[/ITAL]/CDS and [ITAL]TRACE[/ITAL]

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Abstract
Recent observations of solar active regions with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying EUV emission, called the "moss," appearing as a bright dynamic pattern with dark inclusions. The moss has been interpreted as the upper transition region by Berger and coworkers. In this study we use SOHO Coronal Diagnostic Spectrometer 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) × 106 K and is associated with hot loops (T > 2 × 106 K). Moss plasma has an electron density of (2-5) × 109 cm-3 at a temperature of 1.3 × 106 K, giving a pressure of 0.7-1.7 dynes cm-2 (a few times higher than in coronal loops observed in the TRACE Fe IX/X λ171 passband). The volume filling factor of the moss plasma is of order 0.1, and the path along which the emission originates is of order 1000 km long.