Long‐term variations of interplanetary magnetic field spectra with implications for cosmic ray modulation

Abstract

Yearly averaged power spectra of interplanetary magnetic field turbulence at 1 AU are calculated over the period 1965‐1988 for fluctuations in the frequency range 5.8 × 10−6 to 4.6 × 10−5 Hz, corresponding to periods of 6‐48 hours. The spectra are well described by power laws with a typical spectral index of −1.2. Averaged over 24 years, the amplitudes of the component spectra are in the proportion 1.8∶1.5∶1 for magnetic field components, respectively, in the north‐south direction, perpendicular to the Parker spiral in the ecliptic plane, and parallel to the Parker spiral. The amplitudes of the spectra vary with the sunspot cycle and are inversely correlated with the intensity of ∼10‐GeV cosmic rays. Using the observed spectra, we calculate a lower limit to the cosmic ray scattering mean free path using resonant magnetostatic quasi‐linear theory for both “slab” and isotropic geometries of the turbulence. The mean free paths thus obtained are typically ∼0.1 AU in the slab model and ∼0.3 AU in the isotropic model, but they are not significantly correlated with the modulated galactic cosmic ray intensity recorded by neutron monitors. This suggests that the scattering processes described by resonant magnetostatic theory play, at best, a very minor role in the solar modulation of ∼10‐GeV cosmic rays.