The extreme-ultraviolet structure and properties of a newly emerged activeregion

The structure and properties of a newly emerged solar active region (NOAA A ctive Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Sola r and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308-381 Angstrom and 513-633 Angstrom wavelength ranges, while EIT rec orded full-disk EUV images in the He II (304 Angstrom), Fe IX/X (171 Angstr om), Fe xii (195 Angstrom), and Fe XV (284 Angstrom) bandpasses. Electron d ensity measurements from Si rx, Si X, Fe xii, Fe XIII, and Fe xiv line rati os indicate that the region consists of a central high-density core with pe ak densities of the order of 1.2 x 10(10) cm(-3), which decrease monotonica lly to similar to5.0 X 10(8) cm(-3) at the active region boundary. The deri ved electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatu res, and the constant pressure assumption was found to be an unrealistic si mplification. Indeed, the active region is found to have a high-pressure co re (1.3 x 10(16) cm(-3) K) that falls to 6.0 x 10(14) cm(-3) K just outside the region. CDS line ratios from different ionization stages of iron, spec ifically Fe xvi (335.4 Angstrom) and Fe xiv (334.4 Angstrom), were used to diagnose plasma temperatures within the active region. Using this method, p eak temperatures of 2.1 x 10(6) K were identified. This is in good agreemen t with electron temperatures derived using EIT filter ratios and the two-te mperature model of Zhang et al. The high-temperature emission is confined t o the active region core, while emission from cooler (1-1.6) x 10(6) K line s originates in a system of loops visible in EIT 171 and 195 X images. Fina lly, the three-dimensional geometry of the active region is investigated us ing potential field extrapolations from a Kitt Peak magnetogram. The combin ation of EUV and magnetic field extrapolations extends the "core-halo" pict ure of active region structure to one in which the core is composed of a nu mber of compact coronal loops that confine the hot, dense, high-pressure co re plasma while the halo emission emerges from a system of cooler and more extended loops.