High-Resolution Spectroscopy of Gamma-Ray Lines from the X-Class Solar Flare of 2002 July 23

The Reuven Ramaty High Energy Solar Spectroscopy Imager (RHESSI) has obtained the first high-resolution measurements of nuclear de-excitation lines produced by energetic ions accelerated in a solar flare, a GOES X4.8 event occurring on 2002 July 23 at a heliocentric angle of ~73°. Lines of neon, magnesium, silicon, iron, carbon, and oxygen were resolved for the first time. They exhibit Doppler redshifts of 0.1%-0.8% and broadening of 0.1%-2.1% (FWHM), generally decreasing with mass. The measured redshifts are larger than expected for a model of an interacting ion distribution isotropic in the downward hemisphere in a radial magnetic field. Possible interpretations of the large redshifts include (1) an inclination of the loop magnetic field to the solar surface so that the ion distribution is oriented more directly away from the observer and (2) extreme beaming of the ions downward along a magnetic field normal to the solar surface. Bulk downward motion of the plasma in which the accelerated ions interact can be ruled out.