North-South asymmetry of cosmic-ray density gradients throughout the epoch 1955-1991

We have computed the magnitude and direction of the asymmetry of cosmic-ray particle density gradient in the heliosphere during the period 1955-1991. Data obtained by twenty-one detectors (neutron monitors, surface and underground muon telescopes) in both terrestrial hemispheres between 1955 and 1991 are analyzed as a function of the sense of interplanetary magnetic field. Their median rigidity of response (Rm) covers the following range: 10 GV � Rm � 185 GV. Significant differences are frequently observed between the diurnal variations measured in toward and away polarity days. The cosmic-ray density gradient displayed insignificant changes near solar maxima and reversed in sign after the reversal polarity periods. The resultant cosmic-ray gradients are: a north-south symmetric gradient which occurred during minima and maxima solar activity epochs, and a N-S asymmetrical gradient which is related to the N-S asymmetry in the activity on the Sun. Northward and southward cosmic-ray latitudinal (or perpendicular) gradients were frequently observed. The solar diurnal phases of toward polarity days north of the HCS (or away) during the period 1981-87 (qA < 0) existed a few hours later than those recorded for toward (or away) days south of HCS during the positive IMF period 1971-78, as well as the time shift depends on the rigidity of the particle. In addition, quite a change occurred on phase for the north neutron monitors and muon telescopes than for those located on the southern hemisphere