Positions and sizes of X-ray solar flare sources

TL;DR

This study models how Compton backscattering (albedo) affects the apparent positions and sizes of X-ray solar flare sources, revealing significant shifts and size increases that impact observational interpretations.

Contribution

It provides a detailed Monte Carlo simulation demonstrating the albedo effect on X-ray source measurements, highlighting the need to account for this in solar flare analysis.

Findings

Albedo shifts source positions up to 0.5" towards disk center.

Source sizes can double due to albedo, reaching ~7" in 20-50 keV range.

Effect is strongest for flatter spectra, closer sources, and energies 30-50 keV.

Abstract

We investigate the positions and source sizes of X-ray solar flare sources taking into account Compton backscattering (albedo). Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source. We show that the albedo effect will alter the true source positions and substantially increase the measured source sizes. The source positions are shifted up to $\sim 0.5"$ radially towards the disk centre and 5 arcsecond source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. X-ray sources therefore should have minimum observed sizes, thus FWHM source size (2.35 times second-moment) will be as large as $\sim 7"$ in the 20-50 keV range for a disk-centered point source at a height of 1 Mm ($\sim 1.4"$) above the photosphere. The source size and position change is the largest for flatter primary X-ray spectra, stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV. Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and suggest that footpoint sources should be larger in X-rays than in optical or EUV ranges.