Magnetosphere‐ionosphere/thermosphere coupling: Self‐consistent solutions for a one‐dimensional stratified ionosphere in three‐fluid theory

Abstract

We study the local response of a model ionosphere to a change in the magnetospheric convection, on the basis of a three‐fluid (electrons, ions, and neutrals) approach to describing the dynamic processes of solar wind–magnetosphere–ionosphere/thermosphere coupling. The physical description, including the three‐fluid generalized Ohm's law, the plasma momentum equation, and the neutral momentum equation, as well as Maxwell's equations, takes into account electromagnetic coupling among the charged species and collisions among the three species; the geometrical configuration in this initial study, however, is highly simplified and approximates a localized region within the polar cap. We model the driver of the convection by a changing tangential flow of plasma, imposed at the top boundary of the ionosphere, and follow numerically the self‐consistent evolution of the entire system, which is assumed to be incompressible. A magnetic field distortion, corresponding to a horizontal (not field‐aligned) current, propagates from the magnetosphere to the lower ionosphere, producing at first a strong transient Pedersen current which then decreases to a steady state value. The transient time for the system to settle downscales as the Alfvén‐wave travel time between the E layer and the top boundary (verified by redoing the calculations with different heights of the upper boundary). Large perturbations occur during the first 10 Alfvén travel times, and it takes about 20 Alfvén travel times for the system to reach a quasi steady state. After the quasi steady state has been reached, the neutral wind continues to vary slowly (forces due to neutral pressure and effective viscosity have been neglected). When magnetospheric convection is reversed after 1 h, an overshoot of the Pedersen current occurs before the system settles into a new quasi steady state. The electrostatic approximation commonly used in the magnetosphere‐ionosphere coupling models remains poor for up to 10 Alfvén travel times (which could translate to more than 15 min in a more realistic geometry); the assumption that the neutrals remain at rest relative to the Earth is poor within the F layer.