EUV line intensity distribution in the solar atmosphere: differences between a polar coronal hole and its equatorial extension

Abstract

Intensity histograms of 14 EUV emission lines in a polar coronal hole, its equatorial extension, the "Elephant's Trunk", and the adjacent quiet Sun regions have been obtained from SOHO/CDS observations made in August and September 1996. Differences between the quiet region and coronal hole on the one hand, and the polar hole and the equatorial extension on the other, have been examined. As reported earlier, intensity histograms are more similar in the quiet Sun region and the hole for lower transition region lines, while they become increasingly different for upper transition region lines and the corona. However, we also find that coronal holes affect line intensities at most of the atmospheric heights from the transition region to the corona. Histograms of the polar hole and its equatorial extension show dramatic differences with respect to height in the solar atmosphere. The polar coronal hole appears to be brighter than the equatorial extension for emission lines with formation temperature below 0.52 MK, while it appears to be increasingly darker above this temperature. Possible reasons for the polar-equatorial differences in coronal holes are briefly discussed on the basis of flux transport in the solar atmosphere and the consequences of the different orientations of the observed boundary regions of the two coronal holes. The different behaviour seen in the polar coronal hole and its equatorial extension are explained in terms of the effects resulting from different levels of magnetic reconnection occurring in the two regions under study.