Statistically-derived flaring chromospheric-coronal density structure from non-thermal X-ray observations of the Sun

TL;DR

This study uses RHESSI's high-resolution X-ray data to statistically determine the density structure of the Sun's flaring atmosphere, revealing scale heights and ionization transition heights.

Contribution

First statistical determination of the combined chromospheric and coronal density profile during solar flares using high-resolution X-ray observations.

Findings

Density scale height of 131+/-16 km at low altitudes

Density scale height of 5-6 Mm at high altitudes

Transition height between neutral and ionized atmosphere at 1.3+/-0.2 Mm

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.