In-Situ Heating in Active Region Loops

Abstract

We report new observational results and insights in the energy release during transient events on sub-flare level in active region coronal loops. Our work is based on multi-temperature observations obtained high above the limb by the SUMER spectrometer on SOHO. We conclude that the energy is impulsively injected into the loop system from one and only one foot point. This asymmetric injection does not seem to be connected with any bulk flow and there is no indication that the plasma in the loop is replenished or replaced. Therefore the chromospheric evaporation model is not applicable for this type of events. The electron density, N_e, however, increases by up to two orders of magnitude during such events. If no new material is added to the local plasma, then the N_e increase can only be explained by a rapid volume decrease, i.e., by a in-situ pinch effect, compressing and heating the affected plasma.