Observation of a north-south anisotropy of atmospheric radiation at balloon altitudes

Measurements have been made of an azimuthal asymmetry of atmospheric radiation at balloon altitudes and mid-latitudes. The measured asymmetry is 6% peak to peak in the north-south direction at energies above 500 keV. A lack of east-west asymmetry indicates that the arrival direction of the atmospheric radiation is highly decoupled from that of the primary radiation. Present models of secondary atmospheric radiation production and transport do not quantitatively agree with the observations.