Simultaneous balloon observations at Fort Churchill and Minneapolis during the solar cosmic ray events of July 1961

Abstract

The solar cosmic ray events of the period July 12–20, 1961, were studied with many simultaneous balloon flights at Fort Churchill, Manitoba, Canada, and Minneapolis and Bemidji, Minnesota. During the low-energy solar cosmic ray event on July 12–13, γ rays were detected that were produced as the result of nuclear reactions of the solar cosmic rays with air nuclei. The balloon results have been correlated with the Injun 1 results, which showed protons in the range 1 to 15 Mev that could not reach the balloons but that produced the observed γ rays. The observed proton γ-ray correlation agrees well with theoretical calculations of cross sections, and thus we are able with confidence to reconstruct the proton flux between infrequent passes over northern latitudes of Injun 1. This γ-ray inferred time history of the event indicated a gradual increase in proton intensity from about 18 hours before the July 13 sudden commencement until shortly thereafter. Then the fluxes dropped sharply, possibly because of an increased geomagnetic cutoff rigidity at Fort Churchill during the initial phase of the magnetic storm, and showed much time structure during the main phase of the storm. In addition to the γ rays, low-energy X rays were observed coincident with the sudden commencement. These X rays are probably due to electron precipitation, as observed in earlier events. The further events on July 18–20 were studied in their entirety by means of balloons floating at high altitude at Fort Churchill and Minneapolis. Since the complete time history of the solar cosmic rays was obtained, it has been possible to study the mechanisms responsible for the increase and the decay phase of these events. The intensity increase through the time of maximum for particles in a fixed energy region has been found to be well described by the point source infinite medium diffusion equations in interplanetary space. The decay phase, exponential in character, requires the assumption of an escape barrier located at about two astronomical units from the sun. The diffusion mean free path was found to be between 0.025 and 0.04 AU. From diffusion theory, assuming uniform emission from the point source, the number of particles emitted into unit solid angle was found to be about 1032 particles/cm2 sec ster for the event on July 18 and about an order of magnitude lower for the smaller event on July 20. In addition, we may estimate time of ejection of the solar particles from the sun in finding the best fit to diffusion theory. In these events, the simultaneous flights at different latitudes also enabled us once more to study the geomagnetic cutoff changes associated with the magnetic storm periods. The decrease in cutoff was found to correlate well with negative excursions of the equatorial magnetic field, and, because the particle spectrums were known, the cutoffs at Minneapolis could be estimated.