The snowplow in plasmas of nonuniform density

The snowplow motion and the convective skin effect are shown to be the two opposite limits in a unified model of plasma pushing by magnetic pressure. During the snowplow motion the plasma is compressed to a high density in a thin layer and the ion velocity equals the shock velocity. If, on the other hand, the spatial scale of the density gradient is smaller than the ion skin depth, the magnetic field penetrates with a velocity much higher than the ion velocity and the plasma compression is small.