Magnetosphere-ionosphere Energy Transport and Dissipation: 2-D Collisional MHD Simulation Results

In 1996, Eugen Parker called for a paradigm change of magnetospheric physics, more specifically for magnetosphere-ionosphere coupling. The new paradigm rejects the use of the electric field and current as fundamental quantities in magnetosphere-ionosphere coupling. It calls for using the plasma motion and the magnetic field changes as the fundamental variables, the latter of which cannot play a role in the old paradigm of M-I coupling because in all existing global ionospheric simulation models the magnetic field is assumed time-constant although currents are allowed to change, a treatment that may not be good in dynamic processes such as during substorms. The descriptions between the two paradigms are mathematically equivalent, although physically different, for processes greater than 30 min when the magnetosphere-ionosphere system reaches quasi-equilibrium. One of the major differences between the two paradigms is how the magnetospheric energy is transmitted and dissipated in the ionosphere. In the old paradigm, the coupling is via field-aligned currents and/or static electric field and in the new paradigm via wave propagation, penetration, reflection, dissipation and conversion to different modes. Specifically, in the new paradigm, the ionospheric heating rate is determined by the local velocity difference, which may vary in time, between the plasma and neutral wind but not by global field-aligned currents which in steady state are an end effect of large-scale convection. After more than two decades of effort, the theoretical framework has been established and numerical simulation models are under active development. We present the magnetosphere-ionosphere/thermosphere coupling processes described by the new paradigm based on a two-dimensional numerical simulation model. In this model, not only the density, velocity, and temperature for multiple neutral and ion species and electrons, but also the electromagnetic fields are solved in a self-consistent time-dependent manner in the MHD regime, i.e. for processes much slower than the characteristic motion of the electrons. Specifically, the magnetic field is not assumed constant. The 2-D geometry is assumed to describe the situation of the dawn-dusk meridian plane of the global ionosphere driven by an antisunward magnetospheric convection in both polar caps. The simulation shows unambiguously that the ionosphere is driven by the magnetospheric motion via the electromagnetic force and not by the ExB electric drift and that the ionospheric current system is established in a dynamic manner associated with the propagation of the perturbations from the magnetosphere. We will discuss the processes of energy coupling between the magnetosphere and ionosphere and energy dissipation in the ionosphere.