Neutral Effects on Magnetosphere-Ionosphere Coupling

We take a three-fluid approach to describing the dynamic processes of magnetosphere-ionosphere-thermosphere coupling. The three species are electrons, ions, and neutrals. Electromagnetic interactions between the charged species and collisions among the three species are included to derive the three-fluid generalized Ohm's law, the plasma momentum equation, and the neutral momentum equation. The three-fluid generalized Ohm's law describes the time dependence of the current and electron motion, while the plasma (neutral) momentum equation describes the time dependence of the ion (neutral) motion. This formalism adds the neutral momentum equation to the conventional magnetosphere-ionosphere coupling. We derive the general dispersion relation for this three-fluid system and show that at frequencies much below the ion gyrofrequency the phase speed decreases continuously from the Alfven speed as collision frequencies increase at lower altitudes because of neutral-inertia loading . Heavy damping occurs for frequencies higher than 1 Hertz. The magnetosphere-ionosphere coupling is better described as a neutral drag process rather than as resistive Ohmic dissipation.