Determination of Acceleration Time of Protons Responsible for the GLE Onset

Appearance in the solar atmosphere of high-energy protons during major solar flares can be identified from the observation of a broad gamma-ray line in the 70-100 MeV range of the flare emission spectrum. This emission line results from the decay of neutral pions, which, in turn, are produced in interactions of high-energy (> 300 MeV) protons with dense layers of the solar atmosphere. We considered 12 events with clear observations of the pion-decay gamma emission and compared the light curves of this emission with time profiles of different emissions. In 9 events out of 12 we found the onset and peak times of high-energy gamma-rays to be close to the peak times of other electromagnetic emissions and the derivative of the soft X-ray emission. This closeness indicates that efficient acceleration of protons up to sub-relativistic energies starts typically close to the time of the main flare energy release. The further study dealt with the data recorded since 1972 by the world neutron monitor network related to 44 Ground level enhancements (GLEs) and light curves of neutral emissions of the associated flares. The study revealed that a delay of the earliest arrival time of high-energy protons at 1 AU with respect to the observed peak time of the solar bursts did not exceed 10 min in 30 events. This result indicates that in the majority of events, efficient acceleration of protons responsible for the GLE onset should be close to the time of the main energy release in flares.