  The conditions were found when the gaseous medium demonstrates a
unidirectional conductivity on a short time scale; a gas density discontinuity
forms a kind of "gas-dynamic diode" that allows the plasma channel to propagate
in one direction and blocks its development in another. The results of a
two-dimensional numerical simulation of a streamer discharge developing through
a shock wave in air were presented for various neutral density discontinuities
across the wave. The focus was on the case when the streamer propagated from a
low density region to a high-density region. Streamer characteristics changed
greatly after intersecting the shock wave. It was shown that the streamer
failed to penetrate into the high-density region when the ratio between the
densities in these regions was sufficiently high (> 1.2). In this case, the
discharge developed along the surface between these regions after reaching the
boundary between them. Streamers could penetrate into any of the high-density
and low-density regions when a neutral particle density discontinuity was
replaced by a gradual density change.
