COSMIC-RAY MODULATION PARAMETERS DERIVED FROM THE SOLAR DIURNAL-VARIATION

From an analysis of the solar diurnal variation in Galactic cosmic ray with rigidities between 17 and 195 GV from 1957 to 1990, we have deri ved the product of the parallel mean free path and the radial density gradient <(lambda(parallel to G?r))over bar> and the bidirectional lat itudinal gradient indicator G(\z\). We find the rigidity spectrum of t he solar diurnal anisotropy averaged over the entire time span to have a spectral index of -0.1 +/- 0.2 and an upper limiting rigidity of 10 0 +/- 25 GV. The yearly averaged spectrum may depend on the heliospher ic polarity state, having a slightly positive spectral index during th e A > 0 epoch. We find that the parameter <(lambda(parallel to)G(r))ov er bar> depends inversely on rigidity, more so during the A>0 polarity state. Around solar minimum of this epoch, <(lambda(parallel to)G(r)) over bar> is also much lower than for corresponding years in the A < 0 polarity state. Combining the derived values of <(lambda(parallel to G?r))over bar> with results from previous studies of the radial gradie nt, we find evidence for the values of lambda(parallel to) (at all the rigidities examined) having a variation related to the magnetic polar ity state of the heliosphere. G(\z\) indicates that at rigidities up t o 185 GV a bidirectional latitudinal gradient exists and changes direc tion following solar magnetic polarity reversals, in agreement with dr ift theories. This gradient attains its largest magnitudes around time s of solar minimum and also appears to be larger during A < 0 polarity states.