Increased geoeffectiveness of high solar wind dynamic pressure fronts under southward IMF orientation

It is well known that a persistent southward IMF produces increased geomagnetic activity. We have found that southward IMF conditions combined with high solar wind dynamic pressure lead to an even greater coupling between the solar wind and the terrestrial magnetosphere, dramatically increasing the geoeffectiveness of solar wind dynamic pressure events on the Earth. We illustrate this with satellite and ground measurements of various magnetospheric parameters during solar wind pressure front events occurring under steady southward IMF conditions. We show the effect of pressure enhancements on the auroral oval size and precipitation, ionospheric convection, and field-aligned and ionospheric currents. We find an increase in auroral precipitation and a dramatic poleward expansion of the oval and closing of the polar cap at all MLTs following the arrival of a pressure front. A simultaneous enhancement of ionospheric convection all over the polar cap leads to a further increase of an already high cross-polar-cap potential, despite the fact that the magnetosphere is compressed, revealing a substantial increase in the coupling of the solar wind electric field to the Earth's magnetosphere under the high-pressure environment. Finally, an intensification of both field-aligned and ionospheric currents is observed over a wide range of MLTs during the high pressure regime, pointing to increased magnetosphere-ionosphere coupling. This global magnetospheric response to solar wind dynamic pressure enhancements under southward IMF is compared with the weaker response under near-zero IMF B_z prior to the pressure front impact.