Latitudinal dependence of the radial IMF component - interplanetary imprint.

Abstract

Ulysses measurements have confirmed that there is no significant gradient with respect to heliomagnetic latitude in the radial component, B_r_, of the interplanetary magnetic field. There are two processes responsible for this observation. In the corona, the plasma β is <<1, except directly above streamers, so both longitudinal and latitudinal (meridional) gradients in field strength will relax, due to the transverse magnetic pressure gradient force, as the solar wind carries magnetic flux away from the Sun. This happens so quickly that the field is essentially uniform by 5R_sun_. Beyond 10R_sun_, β>1 and it is possible for a meridional thermal pressure gradient to redistribute magnetic flux - an effect apparently absent in Ulysses and earlier ICE and IMP data. We discuss this second effect here, showing that its absence is mainly due to the perpendicular part of the anisotropic thermal pressure gradient in the interplanetary medium being too small to drive significant meridional transport between the Sun and ~4AU. This is done using a linear analytic estimate of meridional transport. The first effect was discussed in an earlier paper.