The time variations of solar cosmic rays during the September 3, 1960, event

This paper describes in detail the results of a series of high-altitude measurements on 17 balloon flights at Fort Churchill, Manitoba, Canada, and Minneapolis, Minnesota, during the flare event of September 3, 1960. This flare is unique in that it is the only one during the solar cycle in which an east-limb event produced both a sea-level and a high-altitude increase in cosmic ray detectors. Detailed investigation of the radio emission identifies this flare as the most probable source of the cosmic ray particles. An X-ray burst observed during the flare is shown to be the result of a geomagnetic disturbance and not to have come directly from the solar surface. The particle onset measured by balloons at high latitude was 83 minutes after the microwave burst associated with flare maximum. The increase of intensity during the first five hours of the event can be fitted well by a diffusion model in interplanetary space with a point source in an infinite diffusing medium. The diffusion analysis shows that the interplanetary main free path on the average is 0.02 astronomical unit. Several interplanetary geomagnetic disturbances, however, distorted the particle increase before maximum intensity was observed. The decay of the event followed an exponential relationship for 140 hours after the flare, with a relaxation time of 23 hours. It is shown that this is consistent with the presence of a barrier at about two astronomical units from which particles escape at infinity and do not return inward. Very complex variations of intensity were observed at Minneapolis during the geomagnetic storm associated with cutoff changes for the incident solar particles in the geomagnetic field. These cutoff changes are well correlated with the main phase equatorial field fluctuations observed. A frequency power spectrum analysis of these time variations shows the presence of an eight-minute period and also longer periods, particularly thirty minutes. The energy spectrum of this event is very flat, which accounts for the particles being observable at sea level even though the high-altitude intensity was not as high as many other events. The spectrum may be well represented by an exponential rigidity for the integral particle flux, and values for the constants are given here. All the balloon data are presented as rate time charts at both the latitudes at which measurements were made.