The time variations of solar cosmic rays during July 1959 at Minneapolis

Thirteen high-altitude balloon flights were made at Minneapolis, Minnesota, during the period July 10–18, 1959. This paper summarizes measurements with ion chambers, Geiger counters, and scintillation counters of the solar cosmic rays accompanying three large flares during that period. Very large fluxes were observed beginning with the main phase of each of the three geomagnetic storms following the flares. During this main phase period, the instrumental results are consistent with protons having a steep energy spectrum and a lower limit at the air cutoff. For example, at 1046 UT on July 15 the integral energy spectrum for protons valid between 88 and 300 Mev was found to be The large increase observed in each storm can be interpreted as a rapid decrease in the cosmic-ray geomagnetic cutoff coincident with the start of the main phase of the storm. The flux returned to normal much sooner than the recovery of the main field of the field of the earth. At the time of the sudden commencement of the geomagnetic storm, a rapid and appreciable decrease of residual low-intensity solar cosmic rays was observed in two cases. It is suggested that these effects may be produced by ring current systems in the outer field of the earth associated with the geomagnetic storm. Following the July 15 increase, intensity oscillations in the solar proton stream with a period of ½ hour were observed. These oscillations are of unknown origin.