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

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 were studied quantitatively. The altitude of the coronal X-ray source was 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 multithermal 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 developed 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 ).

Anomalous Cooling of Coronal Loops With Turbulent Suppression of Thermal Conduction

“…Spatially-resolved soft X-ray observations often show localization of soft X-ray sources near the apex of flaring loops (e.g., Jakimiec et al 1998;Jeffrey et al 2015), which further suggests enhanced trapping of the hot soft-X-ray-emitting plasma. Jiang et al (2006) have investigated the spatial and spectral evolution of such loop-top sources in relation to their cooling properties.…”

The Role of Energy Diffusion in the Deposition of Energetic Electron Energy in Solar and Stellar Flares

“…Current high spectral, spatial and temporal observations with Interface Region Imaging Spectrograph (IRIS ; De Pontieu et al ( 2014)) in the transition region and chromosphere can be used to study the effects of delayed heating by partially or fully thermalized non-thermal electron distributions. Jeffrey et al (2015) show that the flaring corona is made up of multiple loops of varying temperature and density. These varying plasma parameters have a huge effect on both the injected electron parameters and on the resulting energy deposition.…”

Determination of the Total Accelerated Electron Rate and Power Using Solar Flare Hard X-Ray Spectra

“…The residuals show a poorer fit at 15-20 keV. This is likely due to the use of an isothermal approximation for the thermal component (see Jeffrey et al 2015, and the end of Section 4).…”

“…[ For SOL2013-05-13T02:12, the volume was estimated by applying the imaging algorithm Visibility Forward Fitting (VIS FWDFIT; Schmahl et al 2007). Using VIS FWDFIT, the volume of the coronal source varies with energy (see Jeffrey et al 2015), so we determined the mean volume over the energy range of 10-25 keV (Figure 3), yielding a value of V = (0.86 ± 0.20) × 10 27 cm 3 ≈ (0.9 ± 0.2) × 10 27 cm 3 . Using the inferred values of EM 0 and V (and their associated uncertainties) gives n loop = (8.7 ± 2.1) × 10 10 cm −3 .…”

“…We note that an isothermal fit was used to determine the plasma parameters in the flaring corona, even though it has been shown by Jeffrey et al (2015), using a combination of X-ray spectroscopy and imaging, that for SOL2013-05-13T02:12 the coronal source cannot be strictly isothermal. Therefore, the plasma temperature should be viewed as an average temperature throughout the volume in which the electrons propagate.…”