In the Coroniti-Kennel MHD model for the open magnetotail boundary, a standing slow-mode expansion fan turns the magnetosheath flow into the plasma mantle and converts the magnetosheath field into the tail lobe field. The expansion fan-plasma mantle is also called the high-latitude boundary layer. The Coroniti-Kennel model applies to the noon-midnight meridian plane and the case of a purely southward interplanetary magnetic field (IMF). This paper (1) quantitatively tests and verifies the model, and (2) describes how the relatively thick and rapidly propagating high-latitude expansion fan automatically thins and slows at lower latitudes. It tapers down to a thin, nonpropagating tangential discontinuity representing the low-latitude boundary layer on closed field lines. Thus the model shows how the dawn-to-dusk current sheet that bisects the tail horizontally can return to dawn over the tail's back uninterruptedly across a seamless juncture between the high- and low-latitude boundary layers. Other important properties emerge from the complete boundary mode. A rotational discontinuity preconditions the field to pass through the slow mode wave. It induces a secondary circulation in the cross-sectional plane. This circulation sweeps plasma sheet plasma dawnward and duskward away from the tail's midplane and draws it up the tail's flanks, possibly accounting for the plasma sheet's butterfly-shaped cross section. The along-the-tail currents flowing on the rotational discontinuity have the same IMF By dependence as the cusp currents, which they might feed. The By distortion of the secondary circulation tends to rotate the plasma sheet about the tail axis to face the IMF. ¿ American Geophysical Union 1987