We compare the empirical formula, Eq. (1), with
the simulated penetration electric field. In Fig. 5e,
the blue line represents the simulated penetration
electric field, and the magenta line represents the
prediction of the empirical formula using the IEF
Ey in Fig. 5b as input. In the simulations, the CTIPe
model is driven by the high-latitude potential
pattern of Weimer (1996). In Fig. 5e, we plot the
simulated and predicted penetration electric fields,
and the zero lines of the two electric fields coincide
with each other. In other words, all changes of the
electric fields are measured with respect to the zero
lines. The overall agreement between the simulated
and predicted electric fields is very good during the
main phase of the magnetic storm (0400–0800 UT);
the agreement indicates that the IEF continuously
penetrates to the equatorial ionosphere without
decay for the entire interval of southward IMF.
However, the simulations do not show some fast
fluctuations of the IEF. The most likely reason for it
is that time cadence of different inputs driving the
models was not sufficient to resolve these highfrequency
fluctuations. In fact, the high-latitude
inputs to CTIPe (polar cap convection and the
auroral zone particle precipitation and Joule heating)
are updated once every 12 min; in the RCM, the
global magnetospheric magnetic field configuration
driven by the solar wind and other parameters was
updated every 20 min while the polar cap potential
varied with approximately 4 min cadence. Also, the
simulations do not show any negative (westward)
penetration electric fields during northward IMF
after 0800 UT, which may be related to the driving
high-latitude ionospheric convection model used in
the simulations and will be further investigated in a
separate publication.