Bow Shock in the Local Interstellar Medium Ahead of the Heliopause: A Quantitative Analysis

Abstract:

Charge exchange between ions and neutral atoms plays a major role in the solar wind (SW) interaction with the local interstellar medium (LISM). Charge exchange in front of the heliopause is known to produce a hydrogen wall. So-called neutral SW is due to charge exchange of the LISM hydrogen atoms with the SW protons in the supersonic region. Another population of neutral H atoms is born in the inner heliosheath – the region between the heliospheric termination shock and the heliopause. It is well- known that secondary H atoms can affect the LISM flow at large distances from the heliopause. As a result, this flow can become subfast magnetosonic and the conditions necessary for the formation of a bow shock are not satisfied. We present a more quantitative description of this effect using adaptive mesh refinement MHD simulations involving a kinetic treatment of the H atom transport throughout the heliosphere. The main result is that the traditional meaning of a bow shock is no longer valid for realistic LISM properties. More precisely, the contribution of a shock transition to the overall change of plasma quantities across the outer heliosheath (OHS) is minor (20-25%) even for the unperturbed interstellar magnetic field strength of 2.5 microG. For the field strength of 3 microG, the contribution of a shock transition is only a few per cent, while the LISM flow is still superfast magnetosonic. It is therefore clear that no analysis of the unperturbed LISM flow is able to answer the question about the exact contribution of a shock transition. Moreover, for all realistic purposes the distribution of quantities in the OHS may be considered as nearly smooth and only the width of the transition depends on the LISM properties.