Driving mechanism of the nightside ionospheric convection

*Takashi Kikuchi[1]
Communications Research Laboratory[1]

Magnetometer and SuperDARN observations provided evidence of the instantaneous reaction of ionospheric convection on the dayside and nightside. In order to explain the quick response of the nightside ionospheric convection, we examine possible propagation modes, and propose a transmission system composed of the geomagnetic field lines and the Earth-ionosphere waveguide in which the electromagnetic energy is transmitted quickly from the dayside magnetosphere to the nightside ionosphere.

Magnetometer and SuperDARN observations provided evidence of the instantaneous reaction of ionospheric convection on the dayside and nightside. The AMIE analyses revealed that the potential pattern did not move but remained nearly at fixed locations. SuperDARN observations demonstrated that the plasma motion in the nightside ionosphere was intensified immediately after the motion of dayside ionospheric plasma was intensified within a resolution of the measurement (2 min). The convection in the night-side polar ionosphere would cause the plasma convection in the near-earth magnetotail. In the companion paper (Hashimoto and Kikuchi, this meeting) we demonstrate that the growth phase signature at the geosynchronous orbit and the ground magnetic signatures of the partial ring currents developed several minutes after the magnetic reconnection at the dayside magnetopause. These results suggest that the electric field responsible for the convection in the near-Earth magnetotail propagated from the night-side polar ionosphere after having propagated from the magnetosphere to the polar ionosphere on the dayside. In order to explain the quick response of the nightside ionospheric convection, we examine possible propagation modes that could transmit the convection electric field from the dayside outer magnetosphere to the nightside ionosphere. The magnetospheric convection may be generated either by accumulation of the FTEs or by the dynamo action in the cusp and the HLBL. In either case, the electric field propagates from the dayside magnetosphere to the nightside ionosphere within a few minutes. One possible propagation mode would be the magnetosonic wave propagating across the geomagnetic field and the other is the shear Alfven mode propagating parallel to the geomagnetic field. The magnetosonic waves would be totally reflected at the ionosphere and the resultant electric field would be vanished almost completely. On the other hand, the convective motion of the plasma can be driven by shear Alfven waves, but the dynamo in the dayside magnetosphere is not connected directly to the nightside ionosphere by the geomagnetic field lines. In this paper we examine a transmission system composed of the geomagnetic field lines and the Earth-ionosphere waveguide, and conclude that the convection electric field is transmitted from the dayside magnetosphere to the nightside ionosphere through the magnetosphere-ionosphere transmission line system.