Chromospheric height and density measurements in a solar flare observed with RHESSI - II - Data analysis

Aschwanden, M.J., Brown, J.C. and Kontar, E.P. (2002) Chromospheric height and density measurements in a solar flare observed with RHESSI - II - Data analysis. Solar Physics, 210(1-2), 383 -405. (doi: 10.1023/A:1022472319619)

Abstract

We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z (epsilon) as a function of increasing hard X-ray energy epsilon, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy E can be characterized by a power-law function in the epsilon = 15-50 keV energy range, viz., z(epsilon) approximate to 2.3(epsilon/20 keV)(- 1.3) Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in Paper I, which follows the power-law function n(e)(z) = 1.25 x 10(13)(z/1 Mm)(-2.5) cm(- 3). This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h less than or similar to 1000 km, suggesting that the collisional thick- target model is a reasonable first approximation to hard X-ray footpoint sources. At h approximate to 1000-2500 km, the hard X-ray based density model, however, is more consistent with the 'spicular extended-chromosphere model' inferred from radio sub- mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h approximate to 2.5-12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Kontar, Professor Eduard and Brown, Professor John
• Authors: Aschwanden, M.J., Brown, J.C., and Kontar, E.P.
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Solar Physics
• ISSN: 0038-0938