Gamma-rays and neutrons as a probe of flare proton spectra: The solar flare of 11 June 1991

The large flare of 11 June 1991 (GOES class X12) was detected by the Total Absorption Shower Counter (TASC) segment of the EGRET gamma-ray telescope o n board the Compton Gamma Ray Observatory. Significant gamma-ray emission w as observed over the entire energy range to which the TASC was sensitive -1 to 140 MeV. Several phases were identified which showed major changes in t he intensity and spectral shape of the flare gamma-rays. Furthermore, a 'de layed' phase during which a response consistent with the detection of energ etic neutrons and pion-decay gamma-rays was seen, implying a qualitative ch ange in the spectral shape of the accelerated ion spectrum. The similarity of the characteristics of this delayed phase (pion and energetic neutron pr oduction) to those in other large flares hint at a common particle accelera tion mechanism.