Open AccessHigh frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future
Author(s) -
McCracken K. G.
Publication year - 2007
Publication title -
space weather
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.254
H-Index - 56
ISSN - 1542-7390
DOI - 10.1029/2006sw000295
Subject(s) - solar energetic particles , cosmic ray , flare , solar flare , physics , neutron monitor , solar minimum , fluence , astrophysics , atmospheric sciences , event (particle physics) , solar cycle , irradiation , coronal mass ejection , nuclear physics , plasma , solar wind
It is shown that the >4 GeV fluence of large solar energetic particle events was a factor of 10 greater and the frequency of occurrence a factor of four greater prior to 1958 than during the space era. There were two events in 1946 and 1949 with >4 GeV fluences similar to that of 23 February 1956, suggesting that the >4 GeV fluence of the largest probable event for a similar period would be ∼3 times greater. The historic cosmic ray and glaciological records indicate that the fluences and probability of occurrence of such large events at both high and low energies are greatest in periods of low long‐term solar activity and anticorrelated with the estimated strength of the heliospheric magnetic field. A working model is proposed where the factors controlling the occurrence of large SEP events are (1) the occurrence of a large solar flare or CME and (2) the heliomagnetic field being <6.5 nT near Earth. A theoretical basis for this model is discussed. It is proposed that the next Gleissberg minimum of solar activity will lead to a repetition of the pre‐1958 era of high‐frequency, high‐fluence GLE and SEP events. Two predictive indicators of higher SEP activity are proposed: (1) the Climax neutron monitor rate at solar minimum rises >5% compared to 1954 and (2) the measured heliomagnetic field near Earth remains <6.5 nT for ≥2 years into a new solar cycle, or alternatively it is substantially less than 5 nT at sunspot minimum.