Solar cosmic-ray multiply charged nuclei and the July 18, 1961, solar event

Abstract

Energetic helium and heavy nuclei (Z ≥ 3) were detected in the July 18, 1961, solar particle event. This result brings to four the number of solar cosmic-ray bursts in which heavy nuclei have been seen. The particles were detected in nuclear emulsions flown on a balloon launched from Fort Churchill shortly after the associated flare appeared on the sun. The average flux of medium nuclei (6 ≤ Z ≤ 9) during the early part of the solar event was 12.0 ± 1.8 particles m^-2 ster^-1 sec^-1 in the energy interval from 120 to 204-Mev/nucleon, or about 40 times the normal galactic cosmic-ray medium nuclei flux at that particular time in the solar cycle. The helium-to-medium nuclei ratio in the same energy interval was 79 ± 16. These values are consistent with those anticipated on the basis of the relative abundance of hydrogen, helium, and medium nuclei in other events. Some large nuclei (Z ≥ 10) were detected, but light nuclei were so rare that only an upper limit to their abundance could be set (L/M ≤ .07). The unbiased acceleration of multiply charged nuclei in every major solar event now seems more certain, and, therefore, it seems to be worth considering this feature in any theory of solar particle acceleration.