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 Ðeld of view of D60@@] 300@@ and a spatial resolution of D1A. These maps reveal bright features that extend from D6000 K to D2 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 Ðeld of view vary by up to 2 orders of magnitude and have lognormal distributions suggestive of small-scale magnetic fragmentation. We Ðnd empirical evidence for a power-law relationship between line emission and electron density, with an exponent in the range D0.3È1.0. Possible temperature dependence of the exponent is reÑected 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 ÏÏ (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 1A. We also Ðnd that the Li-sequence N V j1240 emission is higher than expected, in agreement with several previous studies. It seems that this e†ect diminishes with intensity, suggesting that it is unlikely to be due to the omission of density dependence in dielectronic recombination calculations. Subject headings : Sun: transition region È Sun: UV radiation