Density and Temperature Measurements in a Solar Active Region

We present electron density and temperature measurements from an active region observed above the limb with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory. Density-sensitive line ratios from Si viii and S x indicate densities greater than 10 cm as high 3 as 200 (or 145 Mm) above the limb. At these heights, static, uniformly heated loop models predict densities close to 10 cm . Differential emission measure analysis shows that the observed plasma is nearly isothermal 3 with a mean temperature of about 1.5 MK and a dispersion of about 0.2 MK. Both the differential emission measure and the Si xi/Si viii line ratios indicate only small variations in the temperature at the heights observed. These measurements confirm recent observations from the Transition Region and Coronal Explorer of “overdense” plasma at temperatures near 1 MK in solar active regions. Time-dependent hydrodynamic simulations suggest that impulsive heating models can account for the large densities, but they have a difficult time reproducing the narrow range of observed temperatures. The observations of overdense, nearly isothermal plasma in the solar corona provide a significant challenge to theories of coronal heating. Subject heading: Sun: corona On-line material: color figure