A&A 489, L57-L60 (2008)
DOI: 10.1051/0004-6361:200810719

Letter

Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop

E. P. Kontar¹, I. G. Hannah¹, and A. L. MacKinnon<sup>2

1</sup>  Department of Physics and Astronomy, University of Glasgow, G12 8QQ, UK
    e-mail: [eduard;iain]@astro.gla.ac.uk
²  Department of ACE, University of Glasgow, G12 8QQ, UK
    e-mail: alec@astro.gla.ac.uk

Received 30 July 2008 / Accepted 26 August 2008

Abstract
Aims. A novel method of using hard X-rays as a diagnostic for chromospheric density and magnetic structures is developed to infer sub-arcsecond vertical variation of magnetic flux tube size and neutral gas density.
Methods. Using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data and the newly developed X-ray visibilities forward fitting technique we find the FWHM and centroid positions of hard X-ray sources with sub-arcsecond resolution (~0.2'') for a solar limb flare. We show that the height variations of the chromospheric density and the magnetic flux densities can be found with an unprecedented vertical resolution of ~150 km by mapping 18-250 keV X-ray emission of energetic electrons propagating in the loop at chromospheric heights of 400-1500 km.
Results. Our observations suggest that the density of the neutral gas is in good agreement with hydrostatic models with a scale height of around 140  30 km. FWHM sizes of the X-ray sources decrease with energy suggesting the expansion (fanning out) of magnetic flux tubes in the chromosphere with height. The magnetic scale height B(z)(dB/dz)^-1 is found to be of the order of 300 km and a strong horizontal magnetic field is associated with noticeable flux tube expansion at a height of ~900 km.

Key words: Sun: chromosphere -- Sun: flares -- Sun: X-rays, gamma rays -- Sun: magnetic fields -- Sun: activity

© ESO 2008

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