DIRECT SOLAR NEUTRONS DETECTED BY NEUTRON MONITORS ON 24 MAY 1990

The ground-level energetic solar particle event on 24 May 1990 has an extremely impulsive onset in the particle intensity that we attribute to a significant flux of direct solar neutrons impacting the top of the atmosphere. This is the largest solar neutron event reported to date with a 25.3% increase recorded by the Climax neutron monitor in the time interval 2050-2055 UT. At least 7 neutron monitors in North America recorded this increase simultaneously, the magnitude of which is ordered by the air mass along the line of sight to the sun at the time of the event. The initial solar neutrons arrive within the same minute as the maximum in both the soft X-ray and the H-alpha emission observed at the earth, whereas the onset of the world-wide increase of solar protons occurred approximately 15 minutes later.