DIURNAL VARIATION IN LOW-ENERGY COSMIC RAY CUTOFFS

Balloon observations are presented of the daily variation in the flux of nuclear γ-rays which were produced by proton bombardment of the terrestrial atmosphere over Byrd Station, Antarctica (L ≈ 7) during the January-February 1967 solar particle events. This modulation is interpreted as the result of a local geomagnetic-time dependence for the cutoff of the solar proton influx at low energies (≈ 1-30 MeV). The observations are compared with predictions of existing theory and, while the predicted effect is confirmed qualitatively, significant discrepancies in quantitative detail are evident. Lack of better agreement lies, most probably, in the assumed simplicity of the magnetospheric models presently in use. © 1969.