Upper limit on the steady emission of the 2.223 MeV neutron capture γ-ray line from the Sun

Abstract

Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) have been searched for evidence of the 2.223 MeV neutron capture line from the Sun, outside the times of γ-ray-emitting solar flares. Background-corrected spectra accumulated over 3-day intervals between 1980 and 1989 show no evidence of the line. Upper limits are reported separately for periods of high and low solar activity.

A conservative 3σ upper limit of 5.7 × 10⁻⁵ γ (cm² s)⁻¹ is placed on the steady flux in the 2.223 MeV line during inactive periods, which is nearly two orders of magnitude lower than previously published results. After correction for limb darkening of the line emission from off-center positions, this upper limit becomes 7.1 × 10⁻⁵ γ (cm²s)⁻¹. Our 3σ upper limit on the steady flux in the line during periods of high solar activity is 6.9 × 10⁻⁵ γ (cm² s)⁻¹, or 8.6 × 10⁻⁵ γ (cm² s)⁻¹ after correction for limb darkening. Our results imply that the quiescent solar corona cannot be heated by ions accelerated above 1 MeV in microflares (or a continuous acceleration process), so long as the ion energy spectrum is similar to that measured in large flares. We also use our results to derive the rate of tritium production at the solar surface; our upper limit of 9 nuclei (cm² s)⁻¹ is about a factor of 9 below the upper limit from searches for ³H in the solar wind. We place upper limits of the order 10³³ on the number of energetic (> 30 MeV) protons which can be stored in active regions prior to being released in solar flares, which imply that the strongest observed flares cannot be produced by such a mechanism.