Bow shock ‘splitting’ in bi‐ion flows

Abstract

This paper discusses modifications of a bow shock ahead of an obstacle in the solar wind (SW) which can occur when the flow consists of a proton plasma and a secondary ion populations. The secondary species may be composed of alpha particles, which are a natural part of the ambient SW, or of heavier particles which are picked up by the solar wind in source regions, such as at comets or Mars. By using a 2D collisionsless bi‐ion fluid model which treats protons and heavy ions as distinct and which assumes that the two fluids communicate with each other by means of electromagnetic forces only, it is shown that for high enough value of the heavy ion mass density a ‘splitting’ of the bow shock takes place. Downstream from the proton bow shock, where differential streaming between ion species arises, a second discontinuity is formed which resembles a shock‐like transition for the heavy ion flow. This plasma boundary, called the heavy‐ion discontinuity (HID), causes also a distinct deflection of the proton flow and significant magnetic field variation. The results seem to be of importance for different types of SW obstacles, especially for planetary objects where massloading of the SW plays a dominant role in bow shock formation, as at comets and probably at Mars. It is suggested that the ‘massloading boundary (MLB)' found in the magnetosheath of Mars and the ‘mysterious boundary’ detected in the cometosheath of Halley and Grigg‐Skejllerup are HID's of the described nature.