Low Mach Number Magnetic Compression Waves in a Collision‐Free Plasma

The development of a strong hydromagnetic disturbance traveling perpendicular to an initially uniform magnetic field in a cold plasma is investigated by numerical integration of the equations of motion. The disturbance is driven by an electric field applied at a fixed plane surface which coincides with the initial boundary of the plasma. If the Mach number of the resulting disturbance is less than two, no crossing of particle orbits occurs. The disturbance then consists of a growing train of almost independent hydromagnetic pulses progressing into the undisturbed plasma at a speed somewhat in excess of the shock velocity which would be calculated from classical theory. The magnitudes of the vacuum magnetic field and the vacuum‐plasma interface velocity are, however, almost identical to the predictions of classical theory. These results, as well as the observed pulse spacing, can be understood in terms of a two‐region model of the disturbed portion of the plasma together with the assumption that the pulses are accelerated by mutual interaction until their spacing substantially exceeds their width.