Analysis of Ground Level Enhancements (GLE): Extreme solar energetic particle events have hard spectra

Abstract

Nearly 70 Ground Level Enhancements (GLEs) of cosmic rays have been recorded by the worldwide neutron monitor network since the 1950s depicting a big variety of energy spectra of solar energetic particles (SEP). Here we studied a statistical relation between the event-integrated intensity of GLEs (calculated as count-rate relative excess, averaged over all available polar neutron monitors, and expressed in percent-hours) and the hardness of the solar particle energy spectra. For each event the integral omnidirectional event-integrated fluences of particles with energy above 30 MeV ($F_{30}$) and above 200 MeV ($F_{200}$) were computed using the reconstructed spectra, and the ratio between the two fluences was considered as a simple index of the event's hardness. We also provided a justification of the spectrum estimate in the form of the Band-function, using direct PAMELA data for GLE 71 (17-May-2012). We found that, while there is no clear relation between the intensity and the hardness for weak events, all strong events with the intensity greater 100 \%*hr are characterized by a very hard spectrum. This implies that a hard spectrum can be securely assumed for all extreme GLE events, e.g., those studied using cosmogenic isotope data in the past.

arXiv commentAdvances in Space Research (in press)

References in corpus (6)

• PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
• The Two Sources of Solar Energetic Particles
• The AD775 cosmic event revisited: the Sun is to blame
• Occurrence of extreme solar particle events: Assessment from historical proxy data
• Observations of the December 13 and 14, 2006, Solar Particle Events in the 80 MeV/n - 3 GeV/n range from space with PAMELA detector
• Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event

Cited by in corpus (4)

• Extreme solar events
• Revisited reference solar proton event of 23-Feb-1956: Assessment of the cosmogenic-isotope method sensitivity to extreme solar events
• Multi-spacecraft observations and transport simulations of solar energetic particles for the May 17th 2012 event
• Solar Energetic Particle Ground-Level Enhancements and the Solar Cycle