Structure of the magnetopause reconnection layer and of flux transfer events: Ion kinetic effects

We have performed large-scale two-dimensional hybrid simulations of reconnection in an asymmetric current sheet applicable to the magnetopause. The ions are treated as macroparticles, and the electrons are a massless charge-neutralizing fluid. We have recovered the essential features of the equivalent MHD simulations, that is, a moving and expanding bulge and a reconnection layer with a high-speed flow of magnetosheath ions on the magnetospheric side. The bulk speed of the magnetosheath ions is twice as high as the speed of the bulge; that is, highspeed flows are predicted on the magnetospheric side of the bulge. A quasi-steady reconnection layer develops behind the bulge in the reconnection outflow region. In the coplanar case the change of the magnetic field from the magnetosheath side to the magnetospheric side takes place at a discontinuity with an electron sense of rotation. This rotational sense is a consequence of the Hall effect near the reconnection Line. Magnetosheath ions exhibit a higher perpendicular temperature on the downstream side of the discontinuity; the discontinuity is therefore most Likely an intermediate shock (IS). The gradient scale length of the IS is of the order of a few magnetosheath ion inertial lengths. In the noncoplanar case (including a B-y interplanetary magnetic field (IMF) component) a rotational discontinuity occurs, which is the minimum shear solution. No other discontinuities, such as slow mode shocks or slow mode waves, have been identified. The high-speed layer is bound to the magnetospheric side by a magnetic boundary with unconnected field Lines. Magnetospheric and magnetosheath ions exhibit the expected velocity filter effects. The two populations penetrate each other without interaction. In the noncoplanar case there are important differences in the structure of the magnetic field and the flow pattern in the high-speed layer. The sense of the rotational discontinuities is determined by the superposition of the magnetic field produced by the Hall current loop and the IMF B-y component. This leads to different rotational senses of the rotational discontinuities on the two sides of the reconnection Line.