STATISTICALLY DERIVED FLARING CHROMOSPHERIC-CORONAL DENSITY STRUCTURE FROM NON-THERMAL X-RAY OBSERVATIONS OF THE SUN

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 data set 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 approximate to 1-1.5 Mm above photosphere), and of 5-6 Mm at high altitudes (corona, above approximate to 2-3 Mm). Assuming a unit step change in ionization level, modeling yields a height of 1.3 +/- 0.2Mm for the transition between fully neutral and 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 similar to 0.5Mm in vertical extent, where neutral densities are beyond 3 x 10(13) cm(-3).