Geomagnetic Measurements on Heavy Primary Cosmic Radiation near the Equator

The azimuthal angular distribution of primary cosmic radiation with charges $Z\ge 6$ has been measured near the geomagnetic equator (at Guam) in horizontal emulsions with known orientation relative to the earth. The observed distribution is well described by using the centered-dipole approximation to the earth's surface magnetic field (north pole at 79°N and 70°W) if the effect of the solid earth is included by assuming that the main cone is the allowed cone. This verifies the prediction that the penumbra becomes forbidden near the geomagnetic equator. The main-cone equatorial cutoffs were applied to an assumed integral energy spectrum which is inversely proportional to the $\alpha$ power of the total energy per nucleon. The exponent $\alpha$ was found to be at least 1.82±0.19.