In a planetary magnetosphere, the bow shock is the boundary at which the solar wind abruptly drops as a result of its approach to the magnetopause. The defining criterion is that the bulk velocity drops from "supersonic" to "subsonic", where these terms have meanings that are specialized to plasma physics. The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which can be seen as the velocity of the points on the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing. A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. ... A magnetopause flows along the boundary between a magnetic field, (see: magnetosphere) and surrounding plasma. ...
The Sun also has a bow shock as it travels through the interstellar medium. A Sun is the star at the center of a solar system. ... The interstellar medium (or ISM) is a term used in astronomy to describe the matter and energy content that exists between the stars (or their immediate circumstellar environment) within a galaxy. ...
The region downstream of the bowshock, between the shock and the magnetopause, that is occupied by the shocked solar wind
That is, the bowshock occurs in a medium--the solar wind--that is so tenuous that collisions among the charged particles that make up the solar wind plasma are exceedingly rare and have no significant influence on the formation of the shock and the dissipation of the solar wind's kinetic energy that occurs there.
Shocks are classified as "quasi-perpendicular" and "quasi-parallel" according to whether the angle (theta) between the interplanetary magnetic field (IMF) upstream of the shock and the shock normal is greater or less than 45 degrees.
The bowshock is the nonlinear wave which stands in the solar wind flow upstream of the Earth's magnetosphere, at which the solar wind plasma is heated and decelerated in preparation for diversion around the magnetosphere.
At the bowshock the magnetic field, density and temperature all increase as the solar wind transits the shock.
Observations of the bowshock show that it is a rich source of waves and energetic particles, and in situ observations have provided something like a plasma laboratory, so that much of the physics of collisionless shocks has been deciphered.
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