Ion Currents, Ion-Neutral Collisions and Plasma Transport Phenomena

Abstract

It is pointed out that, in the commonly encountered plasma configuration of perpendicular electric and magnetic fields, the Pedersen current (in the E direction) is carried by thtl ions, the Hall current (in the E x B direction) is carried by both ions and electrons in nearly equal and mutually canceling parts, and the Birkeland current (in the B direction) is carried by the electrons. One of the consequences of the ion current's existence is that the massive ions are accelerated directly by the fields and not, as in a metallic conductor, through momentum transfer from the electrons. Another is that in the presence of a third neutral component of a partially ionized plasma, again a commonly encountered situation, the electron component of the plasma may be in effect collisionless, the ion component collision-dominated, and the plasma resistivity may then be expected to differ markedly from the resistivity calculated on the basis of Coulomb collisions between electrons and ions. It is shown, for example, that the current structure of an ionizing shock may be adequately explained in terms of an ion-carried current subject to a resistivity, determined solely by ion-neutral collisions, of value two orders of magnitude greater than the Coulomb-collision resistivity. In this, as in the other similarly affected plasma transport phenomenon of diffusion, there is no anomaly nor is there need to invoke plasma instability or fluctuation theory for further explanation.