HARD X-RAY FLARE SOURCE SIZES MEASURED WITH THERAMATY HIGH ENERGY SOLAR SPECTROSCOPIC IMAGER

Abstract

Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations of 18 double hard X-ray sources seen at energies above 25 keV are analyzed to determine the spatial extent of the most compact structures evident in each case. The following four image reconstruction algorithms were used: Clean, Pixon, and two routines using visibilities-maximum entropy and forward fit (VFF). All have been adapted for this study to optimize their ability to provide reliable estimates of the sizes of the more compact sources. The source fluxes, sizes, and morphologies obtained with each method are cross-correlated and the similarities and disagreements are discussed. The full width at half-maximum (FWHM) of the major axes of the sources with assumed elliptical Gaussian shapes are generally well correlated between the four image reconstruction routines and vary between the RHESSI resolution limit of ∼ 2 up to ∼ 20 with most below 10 . The FWHM of the minor axes are generally at or just above the RHESSI limit and hence should be considered as unresolved in most cases. The orientation angles of the elliptical sources are also well correlated. These results suggest that the elongated sources are generally aligned along a flare ribbon with the minor axis perpendicular to the ribbon. This is verified for the one flare in our list with coincident Transition Region and Coronal Explorer (TRACE) images. There is evidence for significant extra flux in many of the flares in addition to the two identified compact sources, thus rendering the VFF assumption of just two Gaussians inadequate. A more realistic approximation in many cases would be of two line sources with unresolved widths. Recommendations are given for optimizing the RHESSI imaging reconstruction process to ensure that the finest possible details of the source morphology become evident and that reliable estimates can be made of the source dimensions.

Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

“…The CLEAN beam factor found for these flares lay in the range 1.9−2.4, suggesting the optimum value is 2 (Dennis & Pernak 2009;Kontar et al 2010), instead of the default value of 1. Figure 1 shows CLEAN maps for the four flares at lowest (10−11.3 keV) and highest (88.6−100 keV) energy bins.…”

“…Since we rely on the source sizes taken from the reconstructed images, we verified the best CLEAN beam width for each flare. Dennis & Pernak (2009) and Kontar et al (2010) have pointed out that CLEAN images usually have systematically larger sizes than other algorithms when using the default beam width factor of 1.0. To ensure the best possible determination of the source sizes using CLEAN images, we applied the visibility forward-fitting procedure (Schmahl et al 2007) on the footpoints of each flare, adjusted the CLEAN beam size, and re-calculated the images until the FWHM of the footpoint from both algorithms had the same size.…”

“…We emphasize that the inference of imaging spectroscopy parameters relies strongly on the precise knowledge of the X-ray source area, which was only developed fairly recently (e.g. Schmahl et al 2007;Kontar et al 2008;Dennis & Pernak 2009;Kontar et al 2010). In addition, the full Spectral Response Matrix (SRM) of RHESSI detectors (including non-diagonal terms) for imaging spectroscopy became available relatively recently, since 2006 February.…”

Positions and sizes of X-ray solar flare sources

“…Kašparová et al 2005). Dennis & Pernak (2009) reported that the average semiminor axis of 18 double source flares is about 4 , while a few of the X-ray source sizes were found to be consistent with line sources along the flare ribbons. While the quantitative comparison with the RHESSI observations requires additional work, we note that zero sizes are either the artifacts of the algorithms used or are caused by the very low source heights.…”

“…Hereafter, following RHESSI measurements (Kontar et al 2008b;Dennis & Pernak 2009;Prato et al 2009) we will refer to the source sizes in terms of FWHM (Full Width Half Maximum), FWHM x,y = 2 √ 2 ln 2σ x,y . The scattered X-ray flux depends on the cosine of the heliocentric angle of the source (μ ≡ cos(θ)) or equivalently on the position of the source at the solar disk, μ = 1 − (x 2 + y 2 )/R 2 .…”