Simultaneous SOHO and Yohkoh observations of a small solar active region

We present new results from observations of a small active region taken wit h the SUMER spectrograph and Extreme Ultraviolet Imaging Telescope (EIT) on SOHO and with the Soft X-Ray Telescope (SXT) on Yohkoh. The SUMER study fe atures line and continuum emission covering a wide range of temperatures an d includes the density-sensitive O IV] lambda 1400 multiplet. A more extens ive analysis of quiet Sun data presented in a previous paper is also includ ed. The presence of a power-law relationship between emission-line power an d electron density is confirmed, although the exponents in the active regio n are slightly higher than those found in the quiet Sun. These power-law re lationships suggest that the volume filling factor decreases with increasin g density and indicate possible differences between emitting material in ac tive regions and the quiet Sun. We study active-region emission measures fr om 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 o f the lower transition region emission-measure distribution for different s ubregions of the data set. This reinforces the conclusion from the original quiet Sun paper that ensembles of "cool loops" are unlikely to be the sour ce of this emission unless the structures are considerably smaller than the 1 " spatial resolution of SUMER.