What is the termination shock?
The termination shock is the boundary marking one of the outer limits of the Sun's influence, and is one boundary of the Solar System. It is where the bubble of solar wind particles slows down so that the particles are traveling slower than the speed of sound. The solar wind particles slow down when they begin to press into the interstellar medium. The solar wind is made of plasma, and when it slows in this way, it goes through many changes. The solar wind plasma gets smooshed together, or compressed like people crowded together in a tiny room. When it is compressed, it also becomes much hotter, in the same way as a bicycle pump heats up in your hand when you vigorously inflate a tire. Also, the solar wind carries outward some of the Sun's magnetic field, which now gets stronger at the termination shock and twists around. We have only two direct measurements of the distance to the Termination Shock. These measurements were made by Voyager 1 and Voyager 2. Voyager 1 crossed the termination shock at 94 astronomical units (AU) and Voyager 2 crossed at 84 AU.
Even though the Interstellar Medium has a low density, it still has a pressure (think of air pressure.) The solar wind flow also represents a strong outward pressure. Close to the Sun, the solar wind has a high pressure and can easily push the interstellar medium away from the Sun. Further away from the Sun, the pressure from the Interstellar Medium is strong enough to slow down and eventually stop the flow solar wind from traveling into space. The place where the speed of the solar wind becomes slower than the speed of sound is called the termination shock.
A similar shock is formed when you run water from a faucet into a sink. When the stream of water hits the sink basin, the flowing water spreads out at a relatively fast speed, forming a disk of shallow water that quickly moves outward, like the solar wind inside the termination shock. Around the edge of the disk, a shock front or wall of water forms; outside the shock front, the water moves relatively slower, like outside the termination shock. Remember, the water shock is only 2-dimensional or flat. The Boundary of our solar system is 3-dimensional like a sphere.
