On the possible acceleration and temporary confinement of energetic ions in the distant heliosphere Original Research Article

Theoretical aspects of the energetic ion motions in the magnetic dip between travelling heliospheric magnetic field perturbations and the termination boundaries of the heliosphere are considered. The group of ions under appropriate initial conditions perform here several bouncing oscillations and could be accelerated along the strongly inclined magnetic fields pointed nearly perpendicular to the radial direction. Accelerating ions are partially confined between the converging magnetic mirrors at the periphery of the heliosphere before escaping in the interstellar medium. We develop a tentative model of these Fermi-type processes which might be useful for the interpretation of recent Voyager 1, 2 observations. The model is capable to reproduce qualitatively the following observed features: (1) the reappearance of MeV proton increases several times during 2000–2004 at both spacecraft because of the solar and heliospheric magnetic field “breathing” due to variations of the solar activity; (2) the time delays between corresponding Voyager 1 and Voyager 2 increases for 3–4 months due to the solar wind propagation; (3) the moderate anisotropy of MeV ion fluxes directed mainly along the magnetic field spirals away from the Sun; (4) the possibility of stronger increases of MeV fluxes at larger heliospheric distances, as was observed in 2003 and 2004, due to the combination of the local acceleration, partial trapping and propagation effects in the variable electric and magnetic fields.