The effect of magnetic helicity on the propagation of galactic cosmic rays

Abstract

Apart from particle drifts, magnetic helicity is the only other known phenomenon that may affect the propagation of galactic cosmic rays in a charge-dependent manner. The detection of steady magnetic helicity in low-frequency interplanetary magnetic field turbulence (Smith and Bieber, 1993) justifies its inclusion in modulation models. In the current model, helicity is introduced through its effect on diffusion parallel to the mean magnetic field. The spatial variation of the parallel mean free path is discussed and it is shown that the largest charge-dependent effect caused by helicity occurs over the solar poles. The transport equation for galactic cosmic rays is solved numerically to illustrate the effect of helicity on particle fluxes. It is shown that helicity causes higher particle intensities during solar cycles with negative polarity and lower intensities during cycles with positive polarity.