The IMF is in general strongly southward during the main phase of extremely large (but rare)
magnetic storms. If the IEF penetrates continuously with an almost fixed efficiency to the middle- and low-latitude ionosphere over a prolonged period, it will cause significant redistribution of the global ionospheric plasma. Huang et al. (2005a) reported the observations of an ionospheric positive storm. In that case, the midlatitude ionospheric F-region electron density, measured by the Millstone Hill incoherent scatter radar, was increased by more
than 100% over 10 h during the daytime, and the radar measurements show that the contribution of
neutral wind disturbances was small. Huang et al. (2005a) concluded that the penetration electric field during the entire 10-h interval of southward IMF is the driving mechanism for the long-duration ionospheric positive storm. The numerical simulations of Swisdak et al. (2006) have successfully reproduced the positive storm and verified the interpretation of Huang et al. (2005a).