Interpretation of Solar Hard X-ray Burst Polarisation Measurements

Abstract

TINDO et al. (refs 1 to 3 and unpublished results) have reported the first observations of polarisation of solar flare emission in the hard X-ray band (around 15 keV) and have made a preliminary interpretation of these³. Their results depend, however, on the removal of systematic differences in sensitivity of the detectors used in the Intercosmos Thomson-scattering polarimeters, by calibration on an ‘unpolarised’ source. We point out here that the results as presented by Tindo et al. are in fact in error since their calibration was based on the assumption that the hard X-ray flux from the flare itself tends to zero polarisation in its final stages of decay. This assumption is invalidated by the presence, in the total flux, of a large polarised contribution from photospheric albedo photons^4,5. We further consider how much this calibration error may affect interpretation of the Intercosmos results in terms of flare particles and suggest how further theoretical work, combined with results from a laboratory calibrated polarimeter, can yield information on the spatial location of hard X-ray flares as well as on their true polarisation.