Introduction

The heliosphere is a vast spheroidal cavity in the local interstellar plasma, some 150–200 AU in size, created by a supersonic flow of plasma called the solar wind, which flows radially outward from the Sun in all directions (Fig. 9.1). The scale of the heliosphere is determined by both the solar atmosphere and the pressure of the surrounding interstellar plasma and magnetic field. Far enough from the Sun the solar wind is spread over such a large volume that it can no longer push back the interstellar plasma. Because the wind is flowing out much faster than waves can propagate inward, the solar wind flow ends at a s pheroidal shock wave, which is called the termination shock, where the supersonic flow changes suddenly to a subsonic outward flow.

The interstellar plasma is moving at about 26 km/s relative to the heliosphere, pushing it in on one side, as shown in Fig. 9.1. Beyond the termination shock, the solar plasma continues to flow outward, but it is deflected and eventually turns to flow in the same direction as the interstellar plasma, forming a large, trailing, heliospheric tail (see Vol. II, Fig. 7.1b). The interstellar medium also contains neutral atoms, and while these play a role in the interaction, they may be neglected in the lowest order.