The dissipation processes at the shock depend on the properties of a collisionless plasma, and lead to a rich range of energetic particles and plasma waves. In an ordinary gas, a shock wave forms when an obstacle is placed in a flow which is supersonic, i.e., the relative speed between flow and obstacle is greater than the sound speed. For example, a supersonic aircraft has an associated shock wave. In a plasma, such as the solar wind, the situation is complicated by the existence of other wave modes in addition to sound waves. However, the principle remains true, and the solar wind flow speed is greater than the sound speed and the Alfven speed, so that a plasma shock is formed, due to the obstacle created by the Earth's magnetosphere.
Shock waves can arise in a number of different situations in astrophysics.
In the case of the magnetized planets in the solar system, the magnetosphere presents an essentially impermeable obstacle, and a steady shock wave is formed which stands at a relatively constant distance from the planet in the upstream flow. This is called a bow shock, in analogy to the bow wave in water ahead of a ship. At the bow shock the magnetic field, density and temperature all increase as the solar wind transits the shock. Thus the bow shock is an example of a fast mode collisionless shock, and the governing characteristic wave speed is that of the fast magnetosonic wave. In an ordinary gas the dissipation necessary at a shock is provided by collisions between the gas molecules, so that the width of the shock is associated with the dissipational scale length which is of the order of the collision mean free path.