RIOMETER OBSERVATIONS AT SANAE (L=4.0) RELATED TO SOLAR PROTON EVENTS

Atmospheric ionization by protons of integral intensity above a cutoff of 0.86 GV at Sanae is too small to record a riometer absorption abov e noise even during the large ground-level enhancements (GLEs) of Sept ember 29 and October 19 and 24, 1989. However, linear increases may be depicted in the absorptions of 20-, 30-, and 51.4-MHz cosmic radio no ise that ranged from 0.25 dB per hour at 30 MHz for a GLE with a risin g phase of 26% per hour to 0.81 dB per hour for a GLE of 36% per hour. Lowering of cutoff rigidity by magnetospheric ring currents to give d irect access to solar flare protons of rigidity below 0.86 GV or preci pitation of energetic magnetospheric electrons due to magnetic activit y may be ruled out. Mutual consistency appears between the rates in gr ound-level solar proton enhancements and the rates of increases in abs orptions at all three of the frequencies for all three events. This su ggests that nonrelativistic solar protons should have traversed the ge omagnetic field lines to be precipitated at Sanae. This traversal may be due to equatorward gradient and curvature drifts from the polar cap region, a westward longitudinal drift along L = 4.0, and subsequent p recipitation into the atmosphere at Sanae due to the South Atlantic Ma gnetic Anomaly. Preceding these linear increases, solar flare effects with absorption maxima coincident with the maxima of solar flare X ray s and the 2.2-MeV gamma ray line have been observed. The rates of reco very in absorption that are faster than that of the 0.5- to 4-angstrom X rays suggest that these absorptions are due to ionization caused by still shorter wavelength electromagnetic radiation.