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X-Ray Polarimetry Workshop SLAC, Stanford, California 9-11 February 2004 |
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RHESSI: solar flare polarimetry
David
Smith
Physics dept. and SCIPP, UC Santa Cruz
dsmith@scipp.ucsc.edu
D. M. Smith, M. L. McConnell, W. Coburn, S. E. Boggs, A. G. Emslie, G. J. Hurford, J. M. Ryan, R. P. Lin
Although designed primarily as a hard X-ray imager and spectrometer, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (30-100 keV) and gamma rays (> 300 keV) from solar flares. The hard x-ray 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 (GeD). The GeDs are segmented, with both a front and rear active volume. Low energy photons (below about 100 keV) can reach a rear segment of a GeD 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 GeD. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates suggest that a 30-100 keV polarization sensitivity of less than a few percent could be achieved for X-class flares. Polarization sensitivity at gamma-ray energies comes from events that Compton scatter from one GeD to another. Since its launch, RHESSI has observed a number of X-class solar flares. We have been analyzing polarimeter mode data of both types from the X4.3 flare that took place on 23 July 2002. There is some evidence for linear polarization in the 20-40 keV energy band, although contributions from systematic effects in the data cannot be completely ruled out. Here we shall review the RHESSI polarimetry capabilities and discuss the analyses of the 23 July 2002 event.
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For more information, contact
Jennifer Formichelli |
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