Different responses of northern and southern high latitude ionospheric convection to IMF rotations: A case study based on SuperDARN observations

We use SuperDARN data to study high-latitude ionospheric convection over a three hour period (starting at 22:00 UT on 2 January 2003), during which the Interplanetary Magnetic Field (IMF) flipped between two states, one with B _y ≫|Bz| and one with B_z >0, both with negative Bx. We find, as expected from previous works, that day side ionospheric convection is controlled by the IMF in both hemispheres. For strongly northward IMF, we observed signatures of two reverse cells, both in the Northern Hemisphere (NH) and in the Southern Hemisphere (SH), due to lobe reconnection. On one occasion, we also observed in the NH two viscous cells at the sides of the reverse cell pair. For duskward IMF, we observed in the NH a large dusk clockwise cell, accompanied by a smaller dawn cell, and the signature of a corresponding pattern in the SH. On two occasions, a three cell pattern, composed of a large clockwise cell and two viscous cells, was observed in the NH. As regards the timings of the NH and SH convection reconfigurations, we find that the convection reconfiguration from a positive B_z dominated to a positive By dominated pattern occurred almost simultaneously (i.e. within a few minutes) in the two hemispheres. On the contrary, the reconfiguration from a B _y dominated to a northward IMF pattern started in the NH 8-13 min earlier than in the SH. We suggest that part of such a delay can be due to the following mechanism: as IMF B_x <0, the northward-tailward magnetosheath magnetic field reconnects with the magnetospheric field first tailward of the northern cusp and later on tailward of the southern cusp, due to the IMF draping around the magnetopause.