4. Conclusions
In this paper, we have performed a statistical
study of observations of penetration of the interplanetary
electric field to the equatorial ionosphere,
using the dayside ionospheric electric field data
obtained by the Jicamarca radar with coherent
scatter mode between 1000 and 1500 LT over five
years (2001–2005). We have derived an empirical
formula of the efficiency of IEF penetration to the
equatorial ionosphere. The efficiency of IEF penetration
to the equatorial ionosphere is found to be
9.6% for rapid southward turnings of the IMF. The
empirical formula can be applied to the cases with
long-duration enhancements of the ionospheric
electric field. The prediction of the empirical
formula is also consistent with the simulation results
of the CTIPe–RCM models, verifying that the
interplanetary electric field can penetrate to the
low-latitude ionosphere for many hours without
obvious decay during the main phase of magnetic
storms as long as the IMF is southward. Figs. 4 and
5 show that the observed/simulated penetration
electric field is in good agreement with the prediction
of the empirical formula over a time interval of
3–4 h during the main phase of magnetic storms
with continuously southward IMF. Note that the
empirical formula is derived from the cases with
rapid IMF southward turnings for which the
shielding effect is supposed to be negligible. The
agreement between the observed/simulated and
predicted electric fields provides evidence that the
IEF can continuously penetrate to the low-latitude
ionosphere for many hours without effective shielding
and supports the conclusion of Huang et al.
(2005b). A very important implication of the
agreement between the empirical formula and
observations/simulations is that the efficiency of
electric field penetration is constant for both rapid
IMF southward turnings and for a prolonged
interval of southward IMF. Therefore the empirical
formula can be used to predict the penetration
electric field within the time interval of several
hours. Our observational results call for an in-depth
theoretical study and extensive numerical simulations
explaining the observed behavior of the
coupled solar wind–magnetosphere–ionosphere–
thermosphere system.