The north-south anisotropy and the cosmic ray radial gradient in the vicinity of the earth

As shown by Bercovitch 1969, an estimate of the cosmic ray radial gradient in the solar system can be obtained from the North-South (NS) anisotropy in the interplanetary space. In the present analysis the neutron monitor data of the polar looking stations Alert (North) and Mc Murdo (South) in the years 1965-68 (i.e. from the minimum to the maximum of solar activity) were used, omitting days of Forbush decreases and days of polarity inversion in the interplanetary magnetic field. The peak-to-peak NS anisotropy which reverses its direction with the field polarity is found to increase from (0.030±0.025)% in solar minimum to (0.30±0.03)% in solar maximum, while the estimated radial gradient increases from (0.4±0.3)%/a.u. up to (4.2±0.4)%/a.u. The implications of this result on the long term modulation mechanisms are discussed taking into account the observed constancy in the solar wind velocity throughout the investigated period.