Directionality of Flare-accelerated -Particles at the Sun

Abstract

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the 7 Be and 7 Li γ-ray lines from fusion of accelerated α-particles with ambient helium produced during the 2002 July 23 solar flare that erupted at a heliocentric angle of 73°. 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 α-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 ∼40° from the normal to the solar surface toward Earth. Such a distribution is also consistent with the α- 4 He line shape with ∼15% confidence.