Nonuniformity of solar protons over the polar caps on March 24, 1966

Observations of ∼10-Mev protons were made over the polar regions by an ionization chamber during two solar proton events on March 24, 1966, which were separated by ∼3.5 hours. Short-term fluctuations in the ionization were observed with factor of 2 changes occurring in ∼30 sec or ∼220 km. A nearly identical ionization chamber at 10–16 RE sunward observed no short-term fluctuations, and these differences are interpreted as spatial effects due to the field structure over the poles. The fluctuations occur mainly when the proton density outside the magnetosphere is increasing rapidly (by a factor of 2 per ∼10 minutes) and when the flux is highly anisotropic. Most of the fluctuations disappear in ∼2 hours, but similarities between the fluctuatons observed during the two solar proton events suggest that the modulation is not interplanetary in origin, and that the magnetospheric field structure causing the modulation retains the same pattern for a period of 3.5 hours. The fluctuations may be due to particles following different paths through the magnetosphere and connecting to different parts of the anisotropic flux behind the bow shock.