Three-Dimensional Distributions of Ionospheric Electric Potentials Determined by a Global MHD Simulation

The ionospheric electric potential is one of the most important parameters for the study of the magnetosphere- ionosphere system since it shows not only the electric field but the convection pattern in geospace system. In recent global MHD simulations and several models of the ionospheric electric potential (e.g. KRM, AMIE, Weimer's model), the ionosphere is treated as a thin layer although the real ionosphere has a three-dimensional structure. To examine 3-dimensional distributions of the ionospheric potential and the current system in the ionosphere, therefore, a solver of the 3-dimensional distribution of the ionospheric electric potential is adopted in M-I coupling process of the global MHD simulation code developed by Tanaka [1995, JGR]. In determining distributions of the electric potentials, field-aligned currents (FACs) on the upper boundary of the ionosphere and 3-dimensional distribution of the ionospheric conductivity are used. As done in most of global MHD simulations, FACs are mapped from the inner boundary of the magnetosphere. As for the ionospheric conductivity, the horizontal distribution of the ionospheric conductivity is determined by the global MHD simulation, including the effects of the solar EUV, diffuse precipitation driven by plasma pressure and temperature, and discreet precipitation modeled by field-aligned current, as described by Tanaka [2000, JGR]. In addition, the height profile of the ionospheric conductivity is proportional to the model distribution determined by IRI 2001. The calculation area covers the polar region which expands 30 degrees of colatitudes. In the present study, a model substorm, which is produced by southward turning of northward IMF, is simulated. Resultant potential patterns in lower ionosphere, where the parallel conductivity corresponds to the Pedersen and Hall conductivities, are similar to the potential patterns determined by the original scheme. The parallel currents mainly connect with the Pedersen currents. On the other hand, the parallel currents also connect with the Hall current in the cusp region and the nightside of the ionosphere, where the distribution of the conductivity is complicated.