Analysis of Ground Level Enhancements (GLE) : extreme solar energetic particle events have hard spectra
Asvestari, E.; Willamo, T.; Gil, A.; Usoskin, I.G.; Kovaltsov, G.A.; Mikhailov, V.V.; Mayorov, A. (2017-08-15)
E. Asvestari, T. Willamo, A. Gil, I.G. Usoskin, G.A. Kovaltsov, V.V. Mikhailov, A. Mayorov, Analysis of Ground Level Enhancements (GLE): Extreme solar energetic particle events have hard spectra, Advances in Space Research, Volume 60, Issue 4, 2017, Pages 781-787, ISSN 0273-1177, http://dx.doi.org/10.1016/j.asr.2016.08.043
© 2016 COSPAR. Published by Elsevier Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/.
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 (F₃₀) and above 200 MeV (F₂₀₀) 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 %∗h 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.
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