Long‐period prompt‐penetration electric fields derived from CHAMP satellite magnetic measurements

Abstract

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 atmospheric sources dominate the EEF at diurnal frequencies, and its subharmonics and magnetospheric sources dominate the EEF for other frequencies. The CHAMP‐derived transfer function has smaller errors than the previous estimates, and we confirm that the average prompt‐penetration response of the equatorial ionospheric electric field to a 1 mV/m change in the IEF is negligible after 3 h and up to the maximum analysis period of 32 h. We update our transfer function model with the new data sets and make the filter coefficients available to the scientific community. The transfer function prediction matched reasonably well with the EEF observation in both the South American and Indian sectors. A transfer function model of the prompt‐penetration effects, driven by the interplanetary electric field, can be highly beneficial to the real‐time specification of the ionosphere.