Directionality of flare-accelerated α-particles at the Sun

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the Be-7 and Li-7 gamma-ray lines from fusion of accelerated alpha-particles with ambient helium produced during the 2002 July 23 solar flare that erupted at a heliocentric angle of 73degrees. This is the first observation of the lines with a germanium spectrometer. We have fitted the measured lines with calculated shapes for different energy and angular distributions for the interacting alpha-particles. A particle distribution from saturated pitch-angle scattering in the corona provides the best fit, but isotropic, downward-isotropic, or fan-beam distributions also provide acceptable fits. We can rule out a downward-beamed distribution with 99.99% confidence. RHESSI measurements of other Doppler-shifted nuclear de-excitation lines are consistent with a forward isotropic distribution of interacting particles in a magnetic loop tilted by similar to40degrees from the normal to the solar surface toward Earth. Such a distribution is also consistent with the alpha-He-4 line shape with similar to15% confidence.