The motion of charged dust particles in interplanetary space - II. Interstellar grains

The effects of electromagnetic forces on charged interstellar grains entering the heliosphere are re-examined. It is shown that the unipolar field regimes at high latitudes lead either to a "focusing" or a "defocusing" of interstellar dust particles with respect to the solar magnetic equator, and that the stochastic magnetic fluctuations in the equatorial region, caused by the warping of the current sheet which separates the polar fields, lead to a diffusive description of particle transport at the low mass end. Consequences for the ability of interstellar dust particles to penetrate the inner heliosphere are discussed. It is concluded that dust particles with radii s > 10 ^-5 cm can penetrate deeply into the heliosphere if their incidence direction at the heliopause is almost radially inward and close to the solar magnetic equatorial plane, whereas dust particles with radii s < 10 ^-5 cm are prevented from reaching the inner heliosphere. This implies that in situ detectors should only be able to observe "core-mantle" particles in the inner heliosphere, but not interstellar "cores". Even this observation should only be possible during favourable solar cycles, when the particles are focused towards the solar magnetic equator.