Bow shock

Bow shocks form the boundary between a magnetosphere and an ambient magnetized medium. This occurs when the magnetic field of an astrophysical object interacts with the nearby flowing ambient plasma. For example, when the solar wind, flowing with a relative speed of order 400 km/s, encounters the magnetic field of Earth, a bow shape boundary forms. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.

The defining criterion of a shock wave is that the bulk velocity of the plasma drops from "supersonic" to "subsonic", where the speed of sound c_s is defined by ${\displaystyle c_{s}^{2}=\gamma p/\rho }$ where ${\displaystyle \gamma }$ is the ratio of specific heats, p is the pressure, and ${\displaystyle \rho }$ is the density of the plasma.

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