ION SUPERDIFFUSION AT THE SOLAR WIND TERMINATION SHOCK

We investigate the propagation of 0.54-3.5 MeV ions accelerated at the termination shock of the solar wind. Data are from Voyager 2 and refer to a time interval about one year long, just before the Voyager 2 termination shock crossing at the end of 2007 August, at roughly 83.7 AU. A recently developed technique, which allows to unravel the transport properties from an analysis of the energetic particle time profiles, is used. The ion time profiles exhibit a power-law decay from a few days to 200 days before the shock front, so that transport is found to be superdiffusive, with a mean square deviation growing like (Δx 2) ∝ t α, with α ∼ 1.3. This means that ion propagation in the heliosphere can be intermediate between normal diffusion and ballistic motion. The implication of ion superdiffusion on particle acceleration mechanisms at the termination shock is discussed, as well as some observational evidence coming from both Voyager 1 and Voyager 2, which questions diffusive shock acceleration.