A formulation providing time variations of the horizontal electric field in the ionosphere from HF Doppler data is presented. The relationship between the electric field and frequency deviations of ionospherically reflected HF radio waves has been extended from the theory given by Jacobs and Watanabe in 1966. In the present formula, the effect of electron decay due to attachment and/or recombination processes is taken into account for long-lasting variations in the electric field. The developed formula can provide time variations of the horizontal field vector even from data observed at only one station. The formula has been applied to an isolated substorm event. In examining the validity of the deduced results, equivalent ionospheric current systems have been used. These have been deduced from worldwide magnetometer data. With these current systems, we can roughly estimate the horizontal electric field in the ionosphere under some assumptions. The time variation of the electric field deduced from the HF Doppler data agrees with that deduced from current systems and high latitude electrojet activity. The results indicate that the HF Doppler method is a sensitive and useful tool for estimating the electric field in the ionosphere.