Constraining Solar Flare Differential Emission Measures with EVE and RHESSI

TL;DR

This paper introduces a new method combining EUV and X-ray spectra from EVE and RHESSI to accurately determine the differential emission measure of solar flares across a wide temperature range, improving understanding of flare energetics.

Contribution

The paper presents the first self-consistent technique to derive a well-constrained DEM for solar flares by combining EUV and X-ray observations, covering the full coronal temperature range.

Findings

Validated the technique on artificial data.

Applied to two X-class flares to determine DEM and its evolution.

Constrained the low-energy cutoff of non-thermal electrons.

Abstract

Deriving a well-constrained differential emission measure (DEM) distribution for solar flares has historically been difficult, primarily because no single instrument is sensitive to the full range of coronal temperatures observed in flares, from $\lesssim$2 to $\gtrsim$50 MK. We present a new technique, combining extreme ultraviolet (EUV) spectra from the EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory with X-ray spectra from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), to derive, for the first time, a self-consistent, well-constrained DEM for jointly-observed solar flares. EVE is sensitive to ~2-25 MK thermal plasma emission, and RHESSI to $\gtrsim$10 MK; together, the two instruments cover the full range of flare coronal plasma temperatures. We have validated the new technique on artificial test data, and apply it to two X-class flares from solar cycle 24 to determine the flare DEM and its temporal evolution; the constraints on the thermal emission derived from the EVE data also constrain the low-energy cutoff of the non-thermal electrons, a crucial parameter for flare energetics. The DEM analysis can also be used to predict the soft X-ray flux in the poorly-observed ~0.4-5 nm range, with important applications for geospace science.