Two-dimensional observations of overshielding during a magnetic storm by the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar

[1] Two-dimensional observations of ionospheric plasma flows possibly caused by overshielding are reported for the first time. The observations were made by the midlatitude Super Dual Auroral Radar Network Hokkaido radar in Japan during a major magnetic storm on 15 December 2006. The magnetosphere was exposed continuously to a southward interplanetary magnetic field (IMF) for several hours during the main phase of the storm. Immediately following the subsequent northward turning of the IMF, an antisunward plasma flow was observed for about 14 min in the predusk sector at magnetic latitudes of 50 degrees-60 degrees, reaching a maximum line-of-sight speed of 70-80 m/s. These features are consistent with a simulation of coupling between the ring current and the ionosphere associated with an overshielding condition. Within 1 h of the first observation, a similar antisunward flow was observed during a period of southward oriented IMF. However, the simulation cannot account for the antisunward flow in this case. It is suggested that the shielding/overshielding condition is not simply caused by the northward turning of IMF. This second overshielding-like condition is attributable to a sudden contraction of the polar cap associated with the substorm or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions. However, neither fully accounts for the observations.