Laboratory measurements indicate that a collisionless shock wave formed in a plasma-wind-tunnel device is nonstationary on the time scale of the ion gyroperiod. This behavior suggests a possible interpretation of recent data on magnetic field structure in the vicinity of Earth's bow shock. Comparison is made between the laboratory measurements and earlier computer simulations in which ion gyromotion was included.

• Received 1 November 1971

DOI:https://doi.org/10.1103/PhysRevLett.28.13