MHD Simulation Study on Quasi-Steady Dawn-Dusk Convection Electric Field in Earth's Magnetosphere

We investigated the large-scale, quasi-steady magnetospheric electric field by using global magnetohydrodynamic (MHD) simulations. When a southward interplanetary magnetic field was imposed, a large-scale, dawn-dusk electric field appeared in the magnetosphere. The dawn-dusk electric field on the dayside is quasi-steady, and can be approximately represented by a scalar potential, that is, an electrostatic field. However, the positive space charge dominates the duskside magnetosphere, while the negative space charge dominates the dawnside magnetosphere. The results suggest the following. (a) At least in the global MHD simulation, space charge accumulation in the magnetosphere alone cannot fully account for the dawn-dusk electric field. Instead, plasma motion plays a primary role in generating the dawn-dusk electric field as previously suggested. (b) Steady convection electric field can be established when plasma flow remains steady. Even under such steady conditions, magnetic energy is continuously transferred from the solar wind to the polar ionosphere, as manifested by integral curves of the Poynting flux vector. This unidirectional energy flow is associated with the convective plasma motion that produces the steady dawn-dusk electric field. (c) Despite the electric field being steady, dynamical processes persist within the magnetosphere. It is also suggested that the magnetosphere basically maintains dynamic equilibrium through a balance of unidirectional energy flow from the solar wind to the ionosphere. When dynamic equilibrium is partially disrupted, the electric field becomes inductive. For the large-scale convection electric field, whether the electric field is electrostatic or inductive depends on the state of dynamic equilibrium.