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 windA stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system. 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 mediumAll the gas and dust found between stars.. The solar wind is made of plasmaPlasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state., 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 fieldA field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force., 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 unitsThe average distance between the Earth and Sun, about 150 million kilometers (93 million miles). (AU) and Voyager 2 crossed at 84 AU.
Even though the interstellar medium has a low densityThe amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm-3. Number density has units cm-3 (particles per cubic centimeter)., 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.