Evidence for charge drift modulation at intermediate solar activity from the flux variation of protons and α particles

We have studied the flux of galactic protons and helium nuclei measured at 1 AU along two consecutive solar activity cycles. We have correlated cosmic ray fluxes measured on IMP 8 satellite at low energy (50 - 300 MeV amu(-1)) with neutron monitor counts registered at Climax station (at energies above few GeV) during the period 1973 - 1995. We have found a systematic excess in the flux of the positive charged particles during the periods with positive solar magnetic field polarity (A > 0) with respect to the flux during the periods of negative polarity (A < 0). This flux excess gives an experimental evidence that charge drift effect plays an important role in the modulation of galactic cosmic rays. A systematic investigation of charge drift modulation is presented. The dependence of the drift effect on the solar activity phase and the particle energy has been also studied. A variation of the proton (as well as helium) flux might be as large as 40%, at the solar activity minimum of two contiguous cycles, at energy less than or equal to 100 MeV.