Hard X-ray solar flare polarimetry with RHESSI
Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20–100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on simulations and in-flight background measurements, indicate that a 20–100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares. The initial results from an analysis of data from the solar flare of 23 July 2002 indicate some modulation of the Be-scattered flux, but whether the modulation arises from polarization effects or whether it represents some instrumental effect is as yet unclear.
Advances in Space Research
Digital Object Identifier (DOI)
M.L McConnell, D.M Smith, A.G Emslie, G.J Hurford, R.P Lin, J.M Ryan, Hard X-ray solar flare polarimetry with RHESSI, Advances in Space Research, Volume 34, Issue 2, 2004, Pages 462-466, ISSN 0273-1177, http://dx.doi.org/10.1016/j.asr.2004.02.007.