Detectability of disturbances in the solar wind

A supersonic disturbance in nonionized air cannot be directly detected by an observer outside the shock wave. The analogous limitation on detection of disturbances in the solar wind is discussed, and the limiting Mach angle is computed in a convenient coordinate system. Recent interpretations of the Imp 1 data (i.e. observation of the moon's wake) are discussed in view of the above restrictions.