Polarization measurements of hard X-rays from solar flares can provide a
direct handle on the extent to which the accelerated electrons are beamed,
which, in turn, has important implications for particle acceleration models.
Past efforts to measured X-ray polarization have met with only limited success.
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 Monte
Carlo 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.
