On Greenstadt's binary index criterion.

The Greenstadt-type criterion for distinguishing between laminar and 'pulsating' (i.e., irregular) bow shock crossings can be expressed as a critical angle between the shock normal and the magnetic field. When the field is more tangent to the shock plane than this critical angle, the shock is laminar. More importantly, the requirement of Galilean invariance of the physics underlying such a criterion reveals that the Greenstadt p index must vary inversely as the solar wind velocity.