A unified theory of cosmic ray diurnal variations

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Abstract
The various observed harmonics of the cosmic ray diurnal variation may be understood on a unified basis if the free space cosmic ray anisotropy is non‐sinusoidal in form. The major objective of this report is to emphasize that such a non‐sinusoidal anisotropy is a natural result of the normal interplanetary transport processes of pitch angle scattering and adiabatic focusing in a spatially varying mean field. Interplanetary scattering parameters λ ∼ 0.5 AU and q ∼ 1.1 at 10 GV rigidity — values that are quite consistent with current knowledge of interplanetary transport processes — lead to predicted semidiurnal and tridiurnal variations that agree with observation. Thus, there is no need to invoke exotic processes to explain the higher order harmonics of the cosmic ray diurnal variation.