Bow shock
A common difficulty in astrophysics is the presence of a magnetic field. For example, the charged particles making up the solar wind follow spiral paths along magnetic field lines. The velocity (speed) of each particle as it moves 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 about the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the part of the velocity parallel to the field lines about which the particles moving in a spiral path.) drops below the speed at which the particles are moving at a spiral path.