Abstract
For the first time, we have used RHESSI's spatial and energy resolution to determine the combined chromospheric and coronal density profile of the flaring solar atmosphere in a statistical manner, using a dataset of 838 flares observable in hard X-rays above 25 keV. Assuming the thick-target beam model, our "average flaring atmosphere" was found to have density scale heights of 131+/-16 km at low altitudes (chromosphere, up to ~1-1.5 Mm above photosphere), and of 5-6 Mm at high altitudes (corona, above ~2-3 Mm). Assuming a unit step change in ionization level, modeling yields a height of 1.3+/-0.2 Mm for the transition between fully neutral to fully-ionized atmosphere. Furthermore, centroids of emission above 50 keV, produced by electrons of similar or higher energies, are located mostly in a small region ~0.5 Mm in vertical extent, where neutral densities are beyond 3x10^13 cm^-3.
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