North-South Asymmetry in Activity on the Sun and Cosmic Ray Density Gradients

摘要

One method of detecting a perpendicular cosmic ray density gradient (vNp) is to make use of the contribution of the drift term B x vNp to the solar cosmic ray diurnal variation. The B x vNp term produces a flow in the ecliptic plane perpendicular to the IMF B, with the sense of the flow depending upon the sense of B; this field-dependent flow adds vectorially to the usual azimuthal streaming which produces a cosmic ray intensity maximum at 18 hours solar time (after correction for geomagnetic bending). In the case of a southward perpendicular gradient, when the IMF is away from the sun, the resultant diurnal variation should have a larger amplitude, and the time of maximum should occur a little earlier, whereas if the IMF is toward the sun, the resultant diurnal variation should have a small amplitude with a later time of maximum. This method is used to analyze neutron monitor and underground muon data, separated according to IMF sense, from 1965 to 1975, to show that there is a perpendicular cosmic ray gradient that pointed southward prior to 1969, and a suggested northward pointing gradient after the reversal of the sun's polar magnetic field in 1969-71.