Limits on Radiative Capture γ-Ray Lines and Implications for Energy Content in Flare-Accelerated Protons

Abstract

Measurements of radiative-capture γ-ray lines can provide information on both the energy content of nonthermal protons below 1 MeV and the temperature in the region where they interact. We have derived upper limits on the fluences in three proton-capture lines of 12C and 13C in the flare-averaged gamma-ray spectrum from 19 X-class flares observed with the Solar Maximum Mission (SMM). The most significant limit comes from the 2.37 MeV line that is excited by 0.46 MeV protons. We estimate an upper limit on the energy content in the accelerated protons by extrapolating the power-law spectrum derived at higher energies down to the resonant energy. The derived upper limit on the temperature, ∼ 5–10×107 K, is higher than measured in the flaring region with other techniques, even for this optimistic energy content. It is possible that NASA's High Energy Solar Spectroscopic Imager (HESSI) will be sensitive enough to detect the 2.37 MeV line if temperatures exceed ∼2×107 K.