FLYWHEEL EFFECT DEDUCED FROM GEOMAGNETIC-VARIATION IN THE POLAR-REGION

The ionospheric neutral wind is induced by the ion drag forcing under the sufficiently continuous southward IMF conditions in the polar regi on. If the IMF turns northward sharply after prolonged southward inter val, the neutral wind gives its own momentum to the charged particles and makes the ionospheric currents. This phenomenon is one of the form s of ''flywheel'' effect. To ascertain the existence of the flywheel e ffect and to obtain its global pattern in the polar region we analyzed the ionospheric equivalent current system derived from ground-based g eomagnetic observation by superposed epoch method. The results show (1 ) appearance of dawnward and antisunward currents after IMF northward turning, (2) its attenuation with a time constant of several hours, (3 ) seasonal dependence in the attenuation time constant, (4) small day- night difference in the current intensity after the turning. The antis unward current is stronger for the case with prolonged southward IMF i nterval before the northward turning than that for short and weak sout hward IMF case. These results are consistent with the theoretical expe ctations of the flywheel effect, though there are some difference with the prediction by computer simulations in the global current pattern such as the dawnward rotation of the current vector.