SUMER observations of the quiet-Sun transition region

We present new results from observations of the quiet Sun taken with the SUMER spectrograph on the SOHO spacecraft. This study includes line and continuum emission covering a wide range of temperatures, and the data have been used to create a number of intensity maps with a field of view of similar to 60 " x 300 " and a spatial resolution of similar to 1 ". These maps reveal bright features that extend from similar to 6000 K to similar to 2 MK, indicating that much of the observed transition region emission arises in an atmospheric interface between the chromosphere and corona. The emission-line intensities over our field of view vary by up to 2 orders of magnitude and have lognormal distributions suggestive of small-scale magnetic fragmentation. We find empirical evidence for a power-law relationship between line emission and electron density, with an exponent in the range similar to 0.3-1.0. Possible temperature dependence of the exponent is reflected in the slope of the emission measure curve, which appears to change slightly with varying electron density. However, there is no major variation in the shape of the lower transition region emission measure distribution with either the intensity or the size of the area studied. This indicates that ensembles of "cool loops" (loops physically distinct from those structures which reach coronal temperatures) are unlikely to be the source of this emission unless the size of the structures is considerably less than 1 " We also find that the Li-sequence N v lambda 1240 emission is higher than expected, in agreement with several previous studies. It seems that this effect diminishes with intensity, suggesting that it is unlikely to be due to the omission of density dependence in dielectronic recombination calculations.