Simultaneous SOHO andYohkoh Observations of a Small Solar Active Region

Details Simultaneous SOHO andYohkoh Observations of a Small Solar Active Region Simultaneous SOHO andYohkoh Observations of a Small Solar Active Region

Jul 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...537..481g&link_type=abstract

The Astrophysical Journal, Volume 537, Issue 1, pp. 481-494.

Astronomy and Astrophysics

Astronomy

6

Sun: Activity, Sun: Transition Region, Sun: Uv Radiation

Scientific paper

We present new results from observations of a small active region taken with the SUMER spectrograph and Extreme Ultraviolet Imaging Telescope (EIT) on SOHO and with the Soft X-Ray Telescope (SXT) on Yohkoh. The SUMER study features line and continuum emission covering a wide range of temperatures and includes the density-sensitive O IV] λ1400 multiplet. A more extensive analysis of quiet Sun data presented in a previous paper is also included. The presence of a power-law relationship between emission-line power and electron density is confirmed, although the exponents in the active region are slightly higher than those found in the quiet Sun. These power-law relationships suggest that the volume filling factor decreases with increasing density and indicate possible differences between emitting material in active regions and the quiet Sun. We study active-region emission measures from both the SUMER and SXT data sets. For the active region as a whole, SXT temperatures and emission measures appear to fit smoothly onto the emission-measure distribution determined from cooler transition region and coronal lines in the SUMER spectra. We find no significant variation in the shape of the lower transition region emission-measure distribution for different subregions of the data set. This reinforces the conclusion from the original quiet Sun paper that ensembles of ``cool loops'' are unlikely to be the source of this emission unless the structures are considerably smaller than the 1" spatial resolution of SUMER.

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