Height structure of X-ray, EUV and white-light emission in a solar flare

TL;DR

This study maps the heights and densities of X-ray, EUV, and white-light sources in a solar flare, revealing their spatial relationships and energy deposition mechanisms in the solar atmosphere.

Contribution

First to determine the heights and densities of multiple emission sources in a solar flare using simultaneous imaging from SDO and RHESSI.

Findings

HXR sources are located 0.8-1.7 Mm above the photosphere.

White-light emission occurs at 1.5-3 Mm, above HXR sources.

EUV emission peaks near 3 Mm, consistent with low-energy electron deposition.

Abstract

The bulk of solar flare emission originates from very compact sources located in the lower solar atmosphere and seen in various wavelength ranges: near optical, UV, EUV, soft and hard X-rays, and gamma-ray emission, yet very few spatially resolved imaging observations to determine the structure of these regions exist. We investigate the above-the-photosphere heights of hard X-ray (HXR), EUV and white-light continuum sources in the low atmosphere and the corresponding densities at these heights. Simultaneous EUV/continuum images from SDO and HXR RHESSI images are compared to study a well observed gamma-ray limb flare. Using RHESSI X-ray visibilities we determine the height of the HXR sources as a function of energy above the photosphere. Co-aligning AIA/SDO and HMI/SDO images with RHESSI we infer, for the first time, the heights and characteristic densities of HXR, EUV and continuum sources in a flaring footpoint. 35-100 keV HXR sources are found at heights between 1.7 and 0.8 Mm above the photosphere, below the white-light continuum emission which appears at heights 1.5-3 Mm, and the peak of EUV emission originating near 3 Mm. The EUV emission locations are consistent with energy deposition from low energy electrons of ~12 keV occurring in the top layers of the fully ionized chromosphere/low corona and not by >20 keV electrons that produce HXR footpoints in the lower neutral chromosphere. The maximum of white-light emission appears between the HXR and EUV emission, presumably in the transition between ionized and neutral atmospheres suggesting free-bound and free-free continuum emission. We note that the energy deposited by low energy electrons is sufficient to explain the energetics of optical and UV emissions.