THE EMISSION MEASURE DISTRIBUTION OF IMPULSIVE PHASE FLARE FOOTPOINTS

Abstract

The temperature distribution of the emitting plasma is a crucial constraint\nwhen studying the heating of solar flare footpoints. However, determining this\nfor impulsive phase footpoints has been difficult in the past due to\ninsufficient spatial resolution to resolve the footpoints from the loop\nstructures, and a lack of spectral and temporal coverage. We use the\ncapabilities of Hinode/EIS to obtain the first emission measure distributions\n(EMDs) from impulsive phase footpoints in six flares. Observations with good\nspectral coverage were analysed using a regularized inversion method to recover\nthe EMDs. We find that the EMDs all share a peak temperature of around 8 MK,\nwith lines formed around this temperature having emission measures peaking\nbetween 10^28 and 10^29 cm^-5, indicating a substantial presence of plasma at\nvery high temperatures within the footpoints. An EMD gradient of EM(T) ~ T is\nfound in all events. Previous theoretical work on emission measure gradients\nshows this to be consistent with a scenario in which the deposited flare energy\ndirectly heats only the top layer of the flare chromosphere, while deeper\nlayers are heated by conduction.\n