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/${\mathrm{cm}}^2$ 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: $\overline{J}=0.44\mathrm{}\pm 0.01, \le 0.50\pm 0.05, \le 0.50\pm 0.05, \mathrm{and} =0.48\mathrm{}\pm 0.01$ ${\left({\mathrm{cm}}^2\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{d}\right)}^{-1\mathrm{}}$. 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.