Physical origin of pickup currents
Abstract. So-called pickup electric currents, associated with the ionization of neutral particles in the presence of a flowing plasma, are conventionally described as produced directly by differential displacement of ions and electrons as the result of acceleration and gyromotion under the action of the electric field E = −V × B∕c. This is not the appropriate physical description for the case when the electron inertial length of the background plasma is small in comparison with the spatial scale of the system. The pickup process in this case does not directly produce a current, except for a small transient on the electron-gyroperiod timescale, which then decays exponentially at the ionization (momentum-loading) rate, as can be shown by an explicit solution of the equations. Rather, the plasma is first slowed down by the momentum loading; the spatially inhomogeneous velocity change then leads to a perturbation of the magnetic field, and the curl of the perturbed field is the current. The timescale for the development of the pickup current is not the ion gyroperiod (as the conventional description might suggest) but rather the Alfvén wave travel time over the spatial scale of the inhomogeneity.