The prompt penetration of the interplanetary electric field to the equatorial ionosphere is conveniently modeled with a frequency‐dependent transfer function. However, long‐period responses (>3 h) of previously estimated transfer functions differ considerably due to insufficient ionospheric eastward electric field (EEF) data useful for spectral analysis. The EEF derived from the Challenging Minisatellite Payload (CHAMP) satellite provides a new opportunity to reliably estimate the long‐period transfer function for the first time. Our objectives in this paper are twofold: first, we analyze the frequency spectra of the equatorial ionospheric eastward electric field for periods greater than 6 h; second, we test the hypothesis that the average prompt‐penetration effect lasts less than 3 h after an initial perturbation in the interplanetary electric field (IEF). We find that atm...