Flow downstream of the heliospheric terminal shock: 1. Irrotational flow

Abstract

Recent reports of remote detection of the heliospheric terminal shock place it near 50 AU. These conflict with standard models which, when combined with current data on the local interstellar medium, place the shock beyond 100 AU. Resolution of this discrepancy has led to hypotheses that invoke cosmic ray pressure, momentum exchange with interstellar neutrals, and magnetic field effects between the shock and the contact discontinuity dividing the solar wind from interstellar plasma. These hypotheses depend not only on properties of the interstellar medium, but also on the downstream three‐dimensional flow between the shock and the contact discontinuity, in the region called the “heliosheath.” We examine the downstream flow field in the absence of magnetic fields under the assumptions that the flow everywhere outside the shock can be approximated as irrotational and incompressible. We find that (1) The distance between the terminal shock and the contact discontinuity is less than the heliocentric distance to the terminal shock, effectively eliminating magnetic field effects in the heliosheath as being dynamically important, (2) the shock should be asymmetric, (3) solar wind flow in the heliosheath is approximately incompressible although it is not well approximated as being irrotational, (4) the external flow in the local interstellar medium can be approximated as incompressible only if the magnetic field contribution to the pressure is considered and if magnetic tension forces are small, and (5) the distant heliotail contains a hot, low density plasma relative to the ambient interstellar medium. Finally, we present plots of the flow topologies and their dependence on flow variables.