A Study of the Frequency of Occurrence of Large-Fluence Solar Proton Events and the Strength of the Interplanetary Magnetic Field |
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Authors: | K G Mccracken G A M Dreschhoff D F Smart M A Shea |
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Institution: | (1) Institute for Physical Science and Technology, University of Maryland, College Park, MD, 20742-2431, U.S.A.;(2) Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, U.S.A.;(3) Emeritus, AFRL (VSBX), Hanscom AFB, Bedford, MA, 01731-3010, U.S.A. |
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Abstract: | It has been shown previously that the number of very-large-fluence solar proton events inferred for the period since 1561
were more frequent at times of low solar activity (e.g., following the recovery from the Maunder minimum), than in the present
epoch of high solar activity. An inverse dependence is demonstrated between the probability of observation of the very large-fluence
solar proton events and the strength of the interplanetary magnetic field derived from empirical predictions. Using the observed
dependence, it is predicted and demonstrated that large-fluence solar proton events have been observed at Earth more frequently
near the recurrent minima of the solar activity cycle in the past than during the present epoch. We show that these results
are explicable in terms of the linear dependence of the Alfvén velocity upon the strength of the interplanetary magnetic field,
leading to higher shock compression ratios in the past. These results indicate that this aspect of “solar weather” will be
significantly influenced by the prevailing strength of the interplanetary magnetic field, and that recurrence of solar conditions
similar to those of the solar activity minimum of solar cycles 12–14 (1878.9–1913.6) would be accompanied by a factor of ∼4
increase in the occurrence of large-fluence solar proton events. |
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