12.5-minute periodicity in solar proton fluxes at balloon altitude and in magnetic micropulsations

Abstract

Quasi‐periodic 12.5‐min fluctuations occurred in the counting rates of Geiger counters and a neutron counter during level flight of a balloon over Wilkes, Antarctica, on September 3, 1966. Correlated fluctuations were observed in the Wilkes magnetometer record. A large solar cosmic‐ray event was in progress at the time of the flight. The periodic modulation process is shown to have an energy dependence with a peak at about 50 Mev for solar protons. No periodicity was detected in the Geiger counter telescope rate, which had a proton threshold energy of > 110 Mev. It is suggested that the modulation is a result of scattering by hydromagnetic waves in the distant part of the tail. The protons with E > 110 Mev did not pass through the modulating region and probably gained access to the magnetosphere on the dayside.