THE MOTION OF IONS IN THE AURORAL IONOSPHERE

The ion motion in the E and F regions relative to the local magnetic f ield line and the horizontal plane has been determined from the Europe an incoherent scatter (EISCAT) common program 1 version I (CP-1-I) dat a. The ion motion highly depends on altitude and magnetospheric electr ic field strength. Under the condition of a strong electric field (\E\ > 25 mV/m), ions move perpendicularly to the local magnetic field lin e both in the F region and at the upper part of the E region (greater than or equal to 117 km). This suggests the ions moving by the ExB dri ft, which is expected when the ion-neutral collision frequency is much smaller than the ion gyrofrequency,At lower altitudes, however, the i ons no longer move perpendicularly to the magnetic field line but move horizontally, as expected from a stronger interaction between the ion s and neutrals. Under the condition of a weak electric field (5 mV/m < \E\ < 10 mV/m) the neutral winds tend to move the ions horizontally e verywhere in the E region and even in the F region. The main cause of these ion motions and their dependence on altitude and electric field strength comes from the relative importance of the magnetospheric elec tric field and the large-scale neutral wind drag to the ionospheric io ns.