Cosmic-Ray Intensity above the Atmosphere at High Latitudes

The total charged particle cosmic-ray intensity above the atmosphere has been measured with thin-walled Geiger counters (total effective stopping power of apparatus and residual atmosphere 0.5 g/cm² of aluminum) carried in balloon-launched rockets at geomagnetic latitudes 54.3^∘{} N, 62.1^∘{} N, 71.9^∘{} N, and 86.7^∘{} N. The respective values of unidirectional particle intensity averaged over the upper hemisphere are: $J̄=0.44±{}0.01, {≤}0.50±{}0.05, {≤}0.50±{}0.05, and =0.48±{}0.01$ ${({cm}²{{0ex}{0ex}}sec{{0ex}{0ex}}sterad)}^{{-}1}. These results are consistent with the complete or nearly complete absence of primary cosmic rays having a magnetic rigidity less than 1.7×{10}⁹$ volts.