THE GLOBAL SOLAR WIND BETWEEN 1 AU AND THE TERMINATION SHOCK

Abstract

This paper studies the global solar wind between 1 AU and the termination shock, taking into account the highly latitude dependent solar wind at the 1 AU boundary and the ionization of interstellar neutral hydrogen. A system of two-fluid magnetohydrodynamics equations governs the flow of solar wind plasma; the conditions of the solar wind at 1 AU are generated from Ulysses data for the relatively stable state of the wind during the declining phase and solar minimum of the solar cycle. A one-fluid model is used to describe the flow of neutral hydrogen and its condition in the far-field boundary outside the termination shock. Simultaneous solutions are obtained using a high-resolution computational code for the two equation systems coupled by the ionization of neutral hydrogen. The ionization process leads to removal of neutral hydrogen in the heliosphere: a hydrogen cavity forms inside ~4 AU; the cavity extends on the downwind side to form a long cavity wake. Solutions show how the ionization process and the wind condition at 1 AU boundary affect the spatial variation of the wind speed, temperature, pickup proton, fast Mach number, and plasma β-ratio. The wind properties inside 4 AU are axisymmetrical about the solar rotation axis; this axi-symmetry totally disappears in the outer heliosphere. All wind properties are substantially modified at an increasing heliocentric distance on the upwind side to generate an upwind-downwind asymmetry dictated by the direction of the relative motion between the Sun and the interstellar medium.