Cloud-chamber observations of solar cosmic rays over Minneapolis on September 4, 1960

The energy spectrum of solar cosmic rays was measured as a function of time during a period of changing geomagnetic cutoff rigidities. A ‘quasi-sharp’ cutoff was present at all times, but the value of the cutoff rigidity changed markedly during the 6-hour flight. No solar cosmic-ray electrons were observed; the upper limit on the ratio of solar electron flux to solar proton flux for particles with momentum >450 Mev/c was 0.25 per cent. The differential energy spectrum of solar protons in the energy range from 160 Mev to 500 Mev was found to be dN/dE = 4.1 × 1010 E−4 peters/Mev for the period 2000 to 2320 UT on September 4, 1960. This spectrum is believed to be essentially the same as that of solar protons in space.