In this paper, we present the statistical result of
the efficiency of IEF penetration to the dayside
equatorial ionosphere. The ionospheric electric field
data are obtained with the Jicamarca radar in
coherent scatter mode. The purpose of this study is
to find how much of the IEF can penetrate to the
equatorial ionosphere and whether the efficiency of
electric field penetration changes with the duration
of southward IMF. We derive an empirical formula
of the penetration efficiency. We also show the