Nature of solar-cycle and heliomagnetic-polarity dependence of cosmic rays, inferred from their correlation with heliomagnetic spherical surface harmonics in the period 1976–1985

Abstract

Correlation of cosmic-ray intensity (I) with the solar magnetic field expanded into the spherical surface harmonics, $\text{B}{}_{\mathrm{n}}\text{s}\text{(n⩽ 9)}$, by Hoeksema and Scherrer has been studied using the following regression equation:

$$\text{I(t)=A}{}_0\text{+}\text{∑}\text{i=1}\text{3}\text{A}{}_{\mathrm{i}}\text{X}\text{(t−τ}{}_{\mathrm{i}}\text{)}$$

, where $\text{X}{}_{\mathrm{i}}\text{s}$ are subgroups of $\text{B}{}_{\mathrm{n}}\text{s}$ classified in ascending order of n, and τ_i is the time lag of I behind correlation coefficient between the observed and simulated intensities (I_obs, I_sml) in the period 1976–1985 is ∼0.87 and considerably better than that derived from any single index of solar activity. The lag time τ₃ is greater than others, indicating that the higher order magnetic disturbances effective to the cosmic-ray modulation have a longer lifetime in space than the lower order disturbances. The rigidity spectrum of the cosmic-ray intensity variation responsible for A_I due to the dipole moment is harder than those for others (A₂,A₃), indicating that the lowest order (i.e. largest scale) magnetic disturbances can modulate cosmic rays more effectively than the higher order disturbances. As another result of the present analysis, it has been found that the intensity depends also on the polarity of the polar magnetic field of the Sun; the residual (I_obs−I_sml) of the simulation changes its sign from positive to negative with a time lag (0–5 Carrington rotation periods) behind the directional change of the solar magnetic dipole moment from northward to southward, and has a softer rigidity spectrum than A_iS. The dependence is consistent with the result having been obtained in the previous period, 1936–1976, by one (K.N.) of the present authors. The polarity dependence can be found also in the 22-year variation of the time lags obtained every solar cycle in the period 1936–1985. The theoretical interpretation of these polarity dependences is discussed on the basis of the diffusion-convection-drift model.