Description
The defining criterion is that the bulk velocity of the fluid (in this case, the solar wind) drops from "supersonic" to "subsonic", where the speed of sound in plasma physics is defined as
where cs is the speed of sound, is the ratio of specific heats, p is the pressure, and is the density of the plasma.
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 is the component of the velocity parallel to the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing.
Bow shocks are also a common feature in Herbig Haro objects, in which a much stronger collimated outflow of gas and dust from the star interacts with the interstellar medium, producing bright bow shocks that are visible at optical wavelengths.
The following images show further evidence of bowshock existence from dense gases and plasma in the Orion Nebula.
Read more about this topic: Bow Shock
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