Using 8 years of ionospheric drift measurements from the low-latitude JULIA (Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere) radar and the solar wind and interplanetary magnetic field data from the ACE (Advance Composition Explorer) satellite, we study the characteristics of the prompt penetration of electric fields to the equatorial ionosphere. A large database allowed us to bring out statistically significant characteristics of electric field penetration as a function of frequency. The coherence between the interplanetary electric field (IEF) and the equatorial electric field (EEF) peaks around a 2-hour period with a maximum magnitude squared coherence of 0.6. The coherence is slightly higher (0.7) on magnetically active (Ap > 20) days. The cross-phase spectra between the ACE and JULIA variations, after elimination of the propagation delay, have negligible values. Correspondingly, the time shift between IEF and EEF is less than 5 minutes at all periods. We also find that the penetration efficiency is highest during local noon, as compared with that of morning and evening hours. The coherence is lower for days with high solar flux values. We find that the penetration of electric fields into the equatorial ionosphere has no significant dependence on season and on the polarity of IMF B _z . We propose a transfer function between IEF and EEF, which was validated on synthetic as well as observed IEF data. The use of this transfer function decreases the misfit of a climatological model with the measured equatorial electric field by 27%.