A possible explanation for rapid, large-scale ionospheric responses to southward turnings of the IMF

We have examined the dayside high-latitude convection response to a sudden southward turning of the Interplanetary Magnetic Field (IMF) and found that the response is nearly instantaneous (<2 min) over a spatial region extend ing from similar to 75 degrees to 85 degrees and from similar to 9 to 16 ML T. Observations of the magnetic field were made with the WIND spacecraft in the solar wind and the GEOTAIL and IMP8 spacecraft in the magnetosheath. I n the high-latitude ionosphere, the HF radars of the Super Dual Auroral Rad ar Network (SuperDARN) were used to monitor the convection. Based on the ma gnetosheath flow of the gas dynamic approximation, the field lines of the n ew IMF state were draped over a large portion of the dayside magnetopause w hen the first significant indication of convection response was measured in the ionosphere. Significant magnetic field line draping accompanied by ext ended reconnection on the dayside magnetopause may help explain the rapid, large-scale response of ionospheric convection.