Creation and expansion of a magnetized plasma bubble for plasma propulsion

We discuss the coupled expansion of a plasma cloud and a neutral gas, both originating from a point-like source located in space and submitted to the action of an external flux of ionizing radiation. This problem is relevant to the artificial magnetospheric propulsion scheme for solar system exploration. We establish the relevant space and time scales for particle diffusion and plasma bubble formation. Emphasis is placed on the low ionization and high collisionallity of the plasma near the point source, leading to the existence of a small ionosphere surrounding the source, where (contrary to the more common views of this propulsion scheme) the ions are not magnetized. The possibility of direct plasma creation by an initial purely neutral gas release is also envisaged.