Letter to the Editor

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

¹ 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 (~) 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 (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.