High frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future

Abstract

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.

Prospects for future enhanced solar energetic particle events and the effects of weaker heliospheric magnetic fields

“…The three LGLEs occurring after 1958 were much smaller than the earlier 5 LGLEs and occurred during relatively low HMF values. McCracken [2007b] suggested that LGLEs occur preferentially when the HMF is ≤6.5 nT, supporting the earlier results [ McCracken et al , 2004] from the lower‐energy LSEP events.…”

“…However, it has long been known [ Cliver et al , 1982; Smart and Shea , 1991] that the first five GLEs, all observed prior to 1958, were far more intense than those that followed. A detailed calibration of the ionization chamber and NM records of two common events [ McCracken , 2007b] has quantified and confirmed that result.…”

The Rough Guide to the Moon and Mars

“…Nor, therefore, is there a simple connection with the corresponding variations in the sunspot number, GCR spectrum and abundance of the 10 Be isotope (figure 8b). We can expect a reversion to the more frequent large SEP events seen just before the space age (McCracken, 2007b). From the inferred fluences of >4 GeV particles, McCracken (2007b) proposes that solar activity must be high enough to give large flares but not so high that the open solar flux is large.…”

Solar Cycle in the Heliosphere and Cosmic Rays

“…Work by McCracken et al (2004) and McCracken (2007b) implies that if Fig. 14 included smaller cycles with SSN cycle values in the range from 0-50, e.g., for the current cycle 24 (Richardson 2013), we might see an anticorrelation between F 200 and SSN cycle .…”