Protons of 0.1 to 5 Mev and electrons of 20 kev at 12 Earth radii during sudden commencement on September 30, 1961

During the period around the sudden commencement of the September 30, 1961, magnetic storm, the ion-electron detector aboard Explorer 12 observed fluxes of low-energy protons and electrons while the satellite was at 12 earth radii and free of the magnetosphere. Whereas the proton intensities initiated an increase about 25 minutes before the sudden commencement, a sudden jump in the omnidirectional intensity occurred eight minutes before the sudden commencement. The maximum flux of about 2×105 protons/cm2 sec ster above 140 kev was attained a half hour after the sudden commencement. Thereafter the intensities varied irregularly with time and displayed fluctuations in anisotropy over the look cone of the detector that were dependent upon the direction of the local magnetic field. The energy spectrum was independent of both time and intensity. The energy density of the proton spectrum (1×10—9 erg/cm3) was too small to account for the observed compression of the earth's field on the sunlit side during the magnetic storm. Time coincident with the sudden jump in proton intensities, an electron flux first appeared and reached its peak value of 3×106 electrons/cm2 sec ster in the 10- to 35-kev range about one minute before the observance of the sudden commencement on the earth's surface. It continued for about 10 minutes and then disappeared. The detection efficiency for electrons places an upper limit of 2×105 electrons/cm2 sec ster for the rest of the storm.