Abstract

It has long been known that large IMF By significantly impacts ionospheric plasma convection and produces asymmetric polar cap potentials. In particular, the plasma circulation known as lobe cell convection is associated with large By and interhemispheric asymmetries, and it is generally attribut... Show moreIt has long been known that large IMF By significantly impacts ionospheric plasma convection and produces asymmetric polar cap potentials. In particular, the plasma circulation known as lobe cell convection is associated with large By and interhemispheric asymmetries, and it is generally attributed to high latitude reconnection. We argue that lobe cell convection may not powered by reconnection, but rather by a direct transfer of mechanical energy extracted from the solar wind at the bowshock into the ionosphere on open field lines via the magnetic shear generated at the bow shock. When the solar wind crosses the bow shock, the transverse components of the magnetic field are compressed. The presence of this magnetic shear means that a large current must flow on the bow shock. For primarily east-west IMF this current flows north-south. The magnetic shear generated at the bow shock extends throughout the magnetosheath. Some of the current from the bow shock flows directly into the polar cap, appearing as an up or down Birkeland current at low altitude. These Birkeland currents drive plasma convection on open field lines as a unipolar generator with opposite polarity in the two hemispheres. We present both simulation results and observations to support our line of argument, and highlight the importance of the bow shock generator in solar wind-magnetosphere energy transfer and the generation of interhemispheric asymmetry. Show less