Perpendicular transport of low-energy corotating interaction region-associated nuclei

We present compelling observational evidence for substantial transport of energetic charged particles across the local average magnetic field. Using data from the STEP/EPACT instrument on board the Wind spacecraft, we find that during three intense corotating interaction region (CIR) events, for periods greater than 12 hr, the observed anisotropy of the particle intensity at 1 AU is often directed at a significant angle to the measured magnetic field direction, which implies significant transport across the local magnetic field. A simple diffusion model is found to fit the three events very well with a large inferred kappa(perpendicular to)/kappa(parallel to). For example, for 80-154 keV nucleon(-1) helium, we find that kappa(perpendicular to)/kappa(parallel to) = 1.47 +/- 0.07, kappa(perpendicular to)/kappa(parallel to) = 0.13 +/- 0.02, and kappa(perpendicular to)/kappa(parallel to) = 0.45 +/- 0.05 for the most intense periods of the three events. We believe that this is the first direct, quantitative measurement in space of large cross-field particle transport, utilizing simultaneous measurements of the streaming particle flux, the solar wind velocity and the magnetic field direction.