The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the star) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. In the Solar System the termination shock is believed to be 75 to 90 astronomical units from the Sun. In 2004, Voyager 1 crossed the Sun's termination shock followed by Voyager 2 in 2007.
The shock arises because solar wind particles are emitted from stars at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. (The exact speed depends on the density, which fluctuates considerably.) The interstellar medium, although very low in density, nonetheless has a constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the star. As one moves far enough away from the star, the pressure from the interstellar medium becomes sufficient to slow the solar wind down to below its speed of sound; this causes a shock wave.
Other termination shocks can be seen in terrestrial systems; perhaps the easiest may be seen by simply running a water tap into a sink creating a hydraulic jump. Upon hitting the floor of the sink, the flowing water spreads out at a speed that is higher than the local wave speed, forming a disk of shallow, rapidly diverging flow (analogous to the tenuous, supersonic solar wind). Around the periphery of the disk, a shock front or wall of water forms; outside the shock front, the water moves slower than the local wave speed (analogous to the subsonic interstellar medium).
Going outward from the Sun, the termination shock is followed by the heliopause, where solar wind particles are stopped by the interstellar medium.
Evidence presented at a meeting of the American Geophysical Union in May 2005 by Dr. Ed Stone suggests that the Voyager 1 spacecraft passed termination shock in December 2004, when it was about 94 AU from the Sun, by virtue of the change in magnetic readings taken from the craft. In contrast, Voyager 2 began detecting returning particles when it was only 76 AU from the Sun, in May 2006. This implies that the heliosphere may be irregularly shaped, bulging outwards in the Sun's northern hemisphere and pushed inward in the south.
The Interstellar Boundary Explorer (IBEX) mission gathered more data on the Solar System's termination shock.
Read more about this topic: Heliosphere
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