Increase in cosmic-ray cutoffs at high latitudes during magnetospheric substorms

Balloon observations obtained at Byrd Station, Antarctica (L ≈ 7), are presented of time variations in the nuclear γ-ray flux produced by proton bombardment of the terrestrial atmosphere during the January 28 and February 2, 1967 solar proton events. The observations reveal evidence for transient increases in the proton cutoff during the local evening and morning hours, and these changes are associated with the occurrence of modest magnetospheric substorms. The increase in the cutoff as observed near earth is delayed by some 10 to 15 minutes from the first appearance of Pi-2 micropulsations. Assuming entry of low-energy solar protons (1–30 Mev) occurs through the magnetotail, the observed delay (mostly of a hydromagnetic origin) suggests that the effective region of entry is located at least 10² RE downstream. Once the cutoff started to increase, an additional 5 to 10 minutes were required for the full effect to take place. It is suggested that the observed delay is the time required for a substantial portion of tail field to collapse toward earth. The cutoff recovered within about an hour, a time scale characteristic of magnetospheric substorm duration, and it is further suggested that this is approximately the time required to re-establish the magnetotail to its former state.