Density and Temperature Measurements in a Solar Active Region

Details Density and Temperature Measurements in a Solar Active Region Density and Temperature Measurements in a Solar Active Region

Oct 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...596l.113w&link_type=abstract

The Astrophysical Journal, Volume 596, Issue 1, pp. L113-L116.

Astronomy and Astrophysics

Astronomy

12

Sun: Corona

Scientific paper

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 108 cm-3 as high as 200" (or 145 Mm) above the limb. At these heights, static, uniformly heated loop models predict densities close to 107 cm-3. Differential emission measure analysis shows that the observed plasma is nearly isothermal 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.

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