Low-energy solar cosmic rays and the geomagnetic storm of May 12, 1959

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During a strong geomagnetic storm on May 11- 12, 1959, and following a class 3+ chromospheric solar flare on May 10, a large intensity of low-energy solar cosmic-ray protons was observed in a balloon flight at 30-km altitude at Minneapolis, Minn. The results obtained with an ion chamber, a Geiger counter, and a scintillation counter are reported. The proton flux of energy above 100 Mev varied from 52 to 1.7/cm/sup 2/ -sec- steradian during the time 0400 to 1800 UT on May 12. The spectrum obtained with the counting instruments is consistent with detailed emulsion studies and has the form N(> E)= CE/sup - s. With E in Mev, gamma varied from 5 to 2 during the time period. The scintillation counter showed excess gamma radiation from the nuclear effects of the cosmic-ray protons and intense bursts of auroral x rays, particularly at 0835 UT on May 12. The solar optical and radio features of the flare are discussed, and the relationship of the polar-cap ionospheric blackout tc the Minneapolis measurements is considered. The cosmic-ray protons at Minneapolis are measured at energies well below the normal Stormer cutoff for that latitude. Several theories on this subject are presented. (auth)