High-temperature differential emission measure and altitude variations in the temperature and density of solar flare coronal X-ray sources

TL;DR

This study uses RHESSI data to analyze the temperature and density structure of solar flare coronal X-ray sources, revealing multi-thermal plasma with altitude-dependent temperature gradients and complex spatial structures.

Contribution

It introduces a method to constrain the differential emission measure (DEM) of flaring plasma using RHESSI imaging, demonstrating the necessity of multi-thermal models for coronal sources.

Findings

Coronal X-ray source altitude increases with energy by ~0.2 arcsec/keV.

Multi-thermal models are required to explain altitude and spectral observations.

Temperature increases with altitude at a gradient of ~0.08 keV/arcsec.

Abstract

The detailed knowledge of plasma heating and acceleration region properties presents a major observational challenge in solar flare physics. Using the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the high temperature differential emission measure, DEM(T), and the energy-dependent spatial structure of solar flare coronal sources are studied quantitatively. The altitude of the coronal X-ray source is observed to increase with energy by ~+0.2 arcsec/keV between 10 and 25 keV. Although an isothermal model can fit the thermal X-ray spectrum observed by RHESSI, such a model cannot account for the changes in altitude, and multi-thermal coronal sources are required where the temperature increases with altitude. For the first time, we show how RHESSI imaging information can be used to constrain the DEM(T) of a flaring plasma. We develop a thermal bremsstrahlung X-ray emission model with inhomogeneous temperature and density distributions to simultaneously reproduce: i) DEM(T), ii) altitude as a function of energy, and iii) vertical extent of the flaring coronal source versus energy. We find that the temperature-altitude gradient in the region is ~+0.08 keV/arcsec (~1.3 MK/Mm). Similar altitude-energy trends in other flares suggest that the majority of coronal X-ray sources are multi-thermal and have strong vertical temperature and density gradients with a broad DEM(T).