Models for Solar Magnetic Loops. IV. On the Relation between Coronal and Footpoint Plasma in Active Region Loops
In the present work we analyze several SUMER intensity maps of active region solar loops in order to compare the relative brightnesses of the footpoints and the coronal section of active region loops. We find that the former are barely distinguishable from the background emission of the active region, while the coronal emission of loops is confined in well-identifiable structures that are significantly brighter than the background. This result means that the vast majority of the active region emission in chromospheric and transition region lines is generated by plasma not directly connected with the coronal plasma that constitutes the observed coronal loops. We determine the observed intensities of coronal lines relative to the observed transition region and chromospheric emission and compare them with predictions from loop models having uniform cross section and different heating functions. We find that the loop models overestimate the footpoint emission by orders of magnitude. We discuss the discrepancy in light of the heating function and of the loop cross section. We speculate that nonuniformity in the loop cross section, more specifically a significant decrease of the cross section near the footpoints, is the most likely solution to the discrepancy.