MHD model merging with IMF By:: Lobe cells, sunward polar cap convection, and overdraped lobes

Sunward convection in the polar cap has long been given as evidence of convection cells that close within the polar cap. These are called "lobe cells" because they are confined to the open field lines that form the tail lobes. Whether lobe-cell convection is physically possible, however, has been seriously questioned. In support of lobe cells, we demonstrate their existence in an MHD model and illustrate the geometry of their circulating magnetic field lines for merging with a purely azimuthal (+B-y) interplanetary magnetic field (IMF). We find that the ratio of flux circulating in lobe cells compared with merging cells increases with decreasing Alfven Mach number and that sunward flow in the polar cap occurs both in lobe cells and on newly opened field lines in normal or "merging" cells. Although at any given time the sunward moving fields are configured in an overdraped pattern similar to that associated with northward IMF, the concept of sunward motion driven by electric field mapping or, equivalently, tugging on these overturned field lines does not apply, at least for the azimuthal IMF case. The overdraped fields and the null points to which they converge are immersed in a diffusion region at the magnetopause and support field-aligned potential drops so that in the course of time, the field lines lose their identity by changing partners. Thus the sunward flowing fields in the ionosphere are decoupled from their magnetopause counterparts and move in response to the ionospheric potential, which has been modified from the applied solar wind potential by diffusion effects. These MHD model results both support and elucidate concepts deduced from primitive superposition and phenomenological models.