Physical Implications of RHESSI Neutron-Capture Line Measurements

Abstract

We report high spectral resolution measurements of the 2.223 MeVneutron-capture line obtained with the Reuven Ramaty High Energy SolarSpectroscopic Imager (RHESSI) from the 2002 July 23 solar flare. Thetime history of this line is affected by both the photospheric3He abundance and the angular distribution of the interactingflare-accelerated particles producing the neutrons. The measured timehistory is compared with predicted time histories calculated using amagnetic loop model with a magnetic field perpendicular to the solarsurface at the footpoints. The model includes energy losses due toCoulomb collisions, removal by nuclear reactions, magnetic mirroring inthe convergent flux tube, and MHD pitch-angle scattering in the corona.This is the first analysis of the neutron-capture line using such aphysically based model. The derived constraints on the interactingangular distribution require that the accelerated particles had tosuffer pitch-angle scattering during their transport through the coronalportion of the loop. The derived photospheric 3He/H ratio wasnot well constrained, primarily because of uncertainties associated withthe measured nuclear de-excitation line flux used to represent theneutron-production time profile.