Trapped protons at low altitudes in the South Atlantic magnetic anomaly

Abstract

The fluxes of protons from 5 to 20 Mev and 60 to 120 Mev were measured in September and October 1962 at low altitudes over the South Atlantic magnetic anomaly. The ratio of the fluxes in these two energy intervals is essentially independent of B and is independent of L below L ≃ 1.4 but changes rapidly for L > 1.4. The flux in the 5- to 20-Mev interval indicates that the spectrum turns back up below the local minimum (at L ≳ 1.5) near 20 Mev. This result is consistent with an increased absorption in the atmosphere for the albedo neutrons near 20 Mev. The integral flux above 31 Mev appears to have increased by a factor of about 3 at the lower L values and higher B values since the Explorer 4 measurements. This would be expected if the source had stayed essentially constant and the atmospheric density had decreased in these regions of space as we moved away from the period of solar maximum.