A technique for the study of short term cosmic ray variations : III) The average diurnal and semidiurnal variations

This paper presents the results of applying the origin-of-scatter technique to determine the relationship between the average diurnal and semidiurnal variations observed on Earth, and the cosmic ray radial gradient and scattering mean free paths in interplanetary space. It is found that the best-fit radial gradient associated with the average diurnal variation near solar maximum has an amplitude of 39/P % per AU when the mean free paths have an r1 dependence. The amplitudes of the best-fit mean free paths are in good agreement with other estimates. The diurnal variation calculated with these best-fit parameters has a rigidity dependence characterized by B= +0.2 for P<25GV and B= -1.0 for P>25GV. The second harmonic of the daily variation associated with these best-fit parameters has an amplitude, phase and rigidity dependence in excellent agreement with the observed semidiurnal variation. It is shown that the semidiurnal variation arises as a natural consequence of the radial dependence of the number density when the mean free paths have an r1dependence. The calculated semidiurnal variation has a rigidity dependence with B=+1.2 for P<30GV, above which the value of B begins to decrease. It is suggested that an ro and an r2 dependence for the long term mean free paths can be excluded because both imply unreasonable semidiurnal variations.