Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic

TL;DR

This paper demonstrates that non-thermal recombination significantly affects flare hard X-ray spectra, especially in hot sources, and must be included for accurate electron diagnostics, challenging the common assumption of bremsstrahlung dominance.

Contribution

It introduces a model including non-thermal recombination onto heavy elements, showing its importance in interpreting HXR spectra from solar flares.

Findings

Recombination onto Fe ions significantly impacts HXR spectra.

Including recombination edges helps diagnose electron cut-off energies.

Recombination can distort inferred electron distributions if neglected.

Abstract

Context. Flare Hard X-Rays (HXRs) from non-thermal electrons are commonly treated as solely bremsstrahlung (f-f), recombination (f-b) being neglected. This assumption is shown to be substantially in error, especially in hot sources, mainly due to recombination onto Fe ions. Aims. We analyse the effects on HXR spectra and electron diagnostics by including non-thermal recombination onto heavy elements in our model. Methods. Using Kramers hydrogenic cross sections with effective Z, we calculate f-f and f-b spectra for power-law electron spectra, in both thin and thick target limits, and for Maxwellians, with summation over all important ions. Results. We find that non-thermal electron recombination, especially onto Fe, must, in general, be included together with f-f, for reliable spectral interpretation, when the HXR source is hot. f-b contribution is greatest when the electron spectral index is large, and any low energy cut-off small. f-b spectra recombination edges mean a cut-off in F(E) appears as a HXR feature at Photon energy = Ec + Vz, offering an Ec diagnostic. Including f-b lowers, greatly in some cases, the F(E) needed for prescribed HXR fluxes and, even when small, seriously distorts F(E) as inferred by inversion or forward fitting based on f-f alone. Conclusions. f-b recombination from non-thermal electrons can be an important contributor to HXR spectra and should be included in spectral analyses, especially for hot sources. Accurate results will require use of better cross sections than ours and consideration of source ionisation structure.