Consideration of a geomagnetic-field aligned electric field E0 in the nonlinear evolution of the cyclotron resonance between electrons and parallel whistler mode waves modifies the conditions required to phase trap energetic particles. Assuming that the deeply trapped, long interacting electrons play the major role in the emissions generated by this nonlinear cyclotron resonance, we utilize a simple phenomenological mechanism to study the influence of the electric field on the ground spectral shapes of equatorial chorus. The alterations brought about by the existence of E0 are sufficiently pronounced to warrant speculations on the potential use of the chorus spectrograms as a diagnostic tool of geomagnetic-field-aligned electric fields, expected to occur, at least temporarily, beyond the plasma-sphere.