SUMER Observations of the Quiet‐Sun Transition Region

Abstract

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 tem- peratures, and the data have been used to create a number of intensity maps with a —eld of view of D60@@ ) 300@@ and a spatial resolution of D1A. These maps reveal bright features that extend from D6000 Kt oD2 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 —eld of view vary by up to 2 orders of magnitude and have lognormal distributions suggestive of small-scale magnetic frag- mentation. We —nd empirical evidence for a power-law relationship between line emission and electron density, with an exponent in the range D0.3¨1.0. Possible temperature dependence of the exponent is re—ected in the slope of the emission measure curve, which appears to change slightly with varying elec- tron 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 ensem- bles of ii cool loops ˇˇ (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 —nd that the Li-sequence N V j1240 emission is higher than expected, in agreement with several previous studies. It seems that this eUect 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 regionSun: UV radiation