The relativistic solar proton ground‐level enhancements associated with the solar neutron events of 11 June, 15 June 1991

The Solar Cosmic Ray Ground‐Level Enhancements (GLEs) observed on 11 and 15 June 1991 were distinctly different in character. The small GLE on 11 June was mildly anisotropic with an aproximately 2‐to‐1 ratio in the relativistic proton flux observed by ‘‘forward viewing’’ high latitude neutron monitors as compared with the flux observed by ‘‘reverse viewing’’ high latitude neutron monitors. In contrast the 15 June GLE was almost isotropic in spite of the fact that the source solar flare position was at heliolongitudes that were presumably ‘‘well‐connected’’ to the earth via the average interplanetary magnetic field topology. A differential power law in rigidity seems to fit the data in the region between 1 and 6 GV for both events. For the 11 June GLE maximum our derived slope is −5.5. For the 15 June GLE maximum our derived slope is −6. It is our opinion that the lack of observed flux anisotorpy during the 15 June GLE is probably due to the very disturbed interplanetary propagation conditions rather than solar source characteristics.