The ion velocity (Bohm–Chodura) boundary condition at the entrance to the magnetic presheath in the presence of diamagnetic and E×B drifts in the scrape-off layer

In the absence of drifts, the Bohm–Chodura criterion gives that the fluid velocity parallel to the magnetic field reaches the ion acoustic speed at the entrance to the magnetic presheath. The changes to this criterion are derived for the situation where poloidal E×B drifts are present. (It was shown earlier that diamagnetic drifts do not influence this criterion.) Here E×B drifts can cause the parallel fluid velocity at the magnetic presheath entrance to become supersonic, subsonic, or reversed (negative).