Detailed analysis of 18 ISEE 1 magnetopause crossings have been made using three-dimensional plasma data obtained by the University of Iowa LEPEDEA's which detect ions and electrons from 1 eV to 45 keV. Crossings were examined that occurred on both the dawn and dusk flanks near the sunward, outer magnetospheric boundary. Nearly all cases show a de-energization of magnetosheath plasma after this plasma crosses the magnetopause to produce the magnetospheric boundary layer. The bulk plasma flow frequently changes direction as it crosses the magnetopause, resulting in boundary layer flow that has an enhanced cross-field flow component without, typically, any increase in speed. These observed flow directions are consistent with Ep¿B drifts imposed by polarization fields Ep established near the forward extent of penetrating magnetosheath irregularities which are producing the boundary layer. Our three-dimensional observations of plasma flow demonstrate that these polarization fields are present and significant. In addition, we directly observe the field-aligned depolarizing currents linked to this dynamo region, which is likely a primary generator for the cusp region and system 1 field-aligned currents. A few exceptional crossings do show plasma energization in the boundary layer, a condition that occasionally results from Maxwell stresses on impulsively injected magnetosheath plasma. However, some prinicpal cases of such energized plasma appear to occur on closed field lines, so that the reconnection mechanism is not necessarily involved in these exceptional crossings. The reconnection hypothesis also predicts that plasma is energized at the expense of field energy density. However, our study demonstrates that the dominant process in the solar wind-magnetosphere interaction results in plasma de-energization as magnetosheath plasma crosses the magnetopause to supply the magnetospheric boundary layer.