Dipole tilt effects on the magnetosphere-ionosphere convection system during interplanetary magnetic field B_Y-dominated periods: MHD modeling

Using numerical magnetohydrodynamic simulations, we examine the dipole tilt effects on the magnetosphere-ionosphere convection system when the interplanetary magnetic field is oblique northward (B_Y = 4 nT and B_Z = 2 nT). In particular, we clarify the relationship between viscous-driven convection and reconnection-driven convection. The azimuthal locations of the two viscous cell centers in the equatorial plane rotate eastward (westward) when the dipole tilt increases as the Northern Hemisphere turns toward (away from) the Sun. This rotation is associated with nearly the same amount of eastward (westward) rotation of the equatorial crossing point of the dayside separator. The reason for this association is that the viscous cell is spatially confined within the Dungey-type merging cell whose position is controlled by the separator location. The ionospheric convection is basically a round/crescent cell pattern, but the round cell in the winter hemisphere is significantly deformed. Between its central lobe cell portion and its outer Dungey-type merging cell portion, the round cell streamlines are deformed owing to the combined effects of the viscous cell and the hybrid merging cell, the latter of which is driven by both Dungey-type reconnection and lobe-closed reconnection.