Observation of Low-Energy Solar Protons During the February 25, 1969 Event

Abstract

This paper reports on initial results from the S71C experiment (University of Bergen, Norway and Danish Space Research Institute, Denmark) pertinent to solar protons. The experiment is flown on board the ESROI (Aurorae) satellite launched on October 3, 1968 into a near-polar orbit with inclination 94°, apogee 1540 km and perigee 260 km. On February 25, 1969 perigee was over the northern polar region. The satellite is magnetically stabilized. The S71C experiment is designed to study trapped and dumped protons in the energy range 100 keV to 6 MeV. The instrument will measure protons belonging to the radiation belt and transient phenomena such as solar and auroral protons. There is an enormous variation both in intensities and spectral hardness of the protons observed during these events. The intensity of solar protons above 100 keV is very small compared with what is normally present above this energy during a proton aurora. Thus, data obtained over a larger part of the polar cap were averaged in order to improve the counting statistics about the solar proton fluxes observed during the February 25 event.