Search for Long-Lived Radioactivity From Solar Flares With RHESSI

Ions accelerated in solar flares interact with the ambient atmosphere to produce not only the prompt gamma- ray line emission that has been observed in many flares, but also radioactive nuclei with long half-lives. The most notable of these nuclei is Co-56, which has a half-life of 78.8~days and upon decay produces very narrow lines at 847~keV and 1.238~MeV. Due to the long half-life of Co-56, the radioactive material will accumulate over multiple flares and persist over many weeks. The initial fluxes seen in these lines can constrain models of ion acceleration in flares in a similar way to fluxes of the prompt lines. More importantly, their decay profile with time would provide an absolutely new probe -- a radioactive tracer -- of the mixing of material from the chromosphere and upper photosphere to greater depths. The high-resolution spectral capabilities of the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) provide the best opportunity to date for detecting such lines above the background. We present the results from our preliminary search for the lines from these long-lived radioactive nuclei. The work at the University of California, Berkeley, was supported by NASA grant NNG05GI89G.