the electric fields during this interval. According to
the definition of the penetration efficiency, the
change of the ionospheric electric field is related to
the change of the IEF. The sudden increases of the
IEF Ey and ionospheric electric field between 1657
and 1806 UT are the changes of the electric fields, so
the minimum value of the predicted penetration
electric field at the beginning of the IMF southward
turning is set to be equal to the minimum value of
the ionospheric electric field at 1657 UT. As seen
from Fig. 3c, the predicted penetration electric field
coincides very well with the measured ionospheric
electric field, indicating that the ionospheric electric
field is indeed the penetration electric field. We also
should mention that this penetration event occurred
during a prolong period of geomagnetic disturbances:
a moderate magnetic storm is evident from
the SYM-H index values (not shown here), with the
peak of the main phase around 03 UT on July 22
(100 nT), and a second decrease in SYM-H
centered around 00 UT on July 25. Thus, the
penetration event occurred well into the recovery
phase. This case was included in the statistics. If we
remove this case from the statistics, the empirical
formula remains the same, and we can use the
empirical formula to predict the penetration electric
field for this case.