The Penumbra at Geomagnetic Latitude 20° and the Energy Spectrum of Primary Cosmic Radiation

The method of obtaining the penumbra presented in a previous paper is applied to the following energies: r=0.385,0.400,0.425,0.450 and 0.500 St\"ormer at a geomagnetic latitude of 20^∘{}. Two graphs showing the variation of the penumbra with the energy are derived from the (γ₁,η) diagrams of these energies: one at a constant zenith angle of 60^∘{}, and the other along the east-west plane. If the number of primaries is assumed to vary inversely as the 2.8 power of their energy, the contribution of the penumbra to the directional intensity at a zenith angle of 60^∘{} is calculated, and is shown to be far from negligible. The calculated intensities are quite sensitive to the energy distribution used, and this suggests a possible method for determining the energy spectrum of primary cosmic rays.