On the current-voltage relationship in fluid theory
Abstract. The kinetic theory of precipitating electrons with Maxwellian source plasma yields the well-known current-voltage relationship (CV-relationship; Knight formula), which can in most cases be accurately approximated by a reduced linear formula. Our question is whether it is possible to obtain this CV-relationship from fluid theory, and if so, to what extent it is physically equivalent with the more accurate kinetic counterpart. An answer to this question is necessary before trying to understand how one could combine time-dependent and transient phenomena such as Alfvénic waves with a slowly evolving background described by the CV-relationship. We first compute the fluid quantity profiles (density, pressure etc.) along a flux tube based on kinetic theory solution. A parallel potential drop accumulates plasma (and pressure) below it, which explains why the current is linearly proportional to the potential drop in the kinetic theory even though the velocity of the accelerated particles is only proportional to the square root of the accelerating voltage. Electron fluid theory reveals that the kinetic theory results can be reproduced, except for different numerical constants, if and only if the polytropic index γ is equal to three, corresponding to one-dimensional motion. The convective derivative term v·∇v provides the equivalent of the "mirror force" and is therefore important to include in a fluid theory trying to describe a CV-relationship. In one-fluid equations the parallel electric field, at least in its functional form, emerges self-consistently. We find that the electron density enhancement below the potential drop disappears because the magnetospheric ions would be unable to neutralize it, and a square root CV-relationship results, in disagreement with kinetic theory and observations. Also, the potential drop concentrates just above the ionosphere, which is at odds with observations as well. To resolve this puzzle, we show that considering outflowing ionospheric ions restores the possibility of having the acceleration region well above the ionosphere, and thus the electron kinetic (and fluid, if γ=3) theory results are reproduced in a self-consistent manner. Thus the inclusion of ionospheric ions is crucial for a feasible CV-relationship in fluid theory. Constructing a quantitative fluid model (possibly one-fluid) which reproduces this property would be an interesting task for a future study.
Key words. Ionosphere (ionosphere-magnetosphere interactions; particle precipitation) · Magnetospheric physics (magnetosphere-ionosphere interactions)