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).