Observations and simulations of the ionospheric and thermospheric response to the December 2006 geomagnetic storm: Initial phase

Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute. The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres.

In the southern hemisphere electron densities decrease slowly with altitude, whereas the decrease is much more rapid in the northern (winter) hemisphere. The electric fields, therefore, cannot cause large increases in electron density by uplifting the plasma, so positive storm effects are small in the southern hemisphere compared with the northern hemisphere, even though the increase in hm F₂ is greater in the southern hemisphere.

Nighttime changes in electron density in other longitude sectors are small, because the topside electron densities also decrease slowly with altitude at night.