Relation between cusp and mantle in MHD simulation

A global MHD simulation illustrates the relative contributions to magnetotail structure of two mantle models. One model is the data‐based picture that Rosenbauer et al. [1975] originally put forth showing the cusp to be the source of the mantle. In this model the cusp is populated by magnetosheath particles that enter it as a consequence of magnetic reconnection on the day side magnetopause. The other model, which derives from MHD considerations, notes that since the mantle lies on open magnetic field lines, the solar wind should be able to enter it along the tail magnetopause downwind from the cusp. In the simulation results presented here, both modes of plasma entry into the mantle occur and their relative contributions are seen. The mantle, identified in the simulation as the volume of the tail occupied by a slow‐mode expansion wave filled with magnetosheath plasma, lies mostly on streamlines that originate in the dayside cusp (cusp model), but the bulk of plasma inflow comes directly from the magnetosheath (magnetosheath model). The demarcation between streamlines originating in the cusp (cusp model) and streamlines originating in the solar wind (magnetosheath model) is sharp. We call this demarcation the fluopause. The fluopause is the downwind extension of the solar wind stagnation streamline. At 100 R E downtail the fluopause lies several R E inside the magnetopause identified by the peak in the electrical current that defines the magnetotail boundary. The spatial separation between the fluopause and the magnetopause defines the volume to which the magnetosheath model refers. Whereas the magnetopause clearly marks the start of the slow‐mode expansion wave that defines the mantle, all MHD parameters and their slopes are continuous across the fluopause. The fluopause is only determinable by following streamlines. Streamlines in the mantle that originate in the solar wind (magnetosheath model) do not converge on the plasma sheet and, so, are not a likely source of particles for the plasma sheet. Streamlines in the mantle that originate in the cusp (cusp model) do converge on the plasma sheet and, so, are likely candidates for the particles that supply the plasma sheet. At their earthward ends, streamlines that originate in the cusp reach low altitudes and might, in nature, be populated with ionospheric plasma. An implication of the present result is that the inner portions of mantle encounters that have been identified in IMP 8 and Geotail data refer to plasma that has spent part of its life in the dayside cusp and possibly in the ionosphere. The latter possibility might account for Geotail observations that identify O + as a common constituent of deep‐mantle plasma.