UL components for the substorms in March 17–19, 1978. However, equivalent currents are not real currents that can be detected. The separated DD and UL components are ambiguous in physics and cannot be confirmed by observations. [3] Statistical observations of the ionospheric electric field showed that in the auroral zone midnight sector the southward electric field (Es) suddenly enhanced up to � 30 mV/m from 0 and the westward electric field (Ew) jumped only � <5 mV/m at substorm expansion phase onset [Mozer, 1971]. The Es magnitude exceeded 30 mV/m, which was significantly higher than the increase of the Ew during substorm growth phase. Kamide and Kokubun [1996] also suggested that there are two major components in the ionospheric electric field, the Ew associated with magnetospheric convection and the Es associated with a substorm expansion phase. Kan [2007] proposed that an Es can be formed due to a blockage of the northward Hall currents produced by the Ew. The enhanced Es drives an intense westward Hall current in the ionosphere, i.e., the substorm westward electrojet. Its closure current in the plasma sheet may play an important role in causing a reduction and/or a disruption of the cross-tail current as well as a dipolarization of the magnetic field. Therefore, a quantitative description of Es and Ew is very important in understanding the development of substorms, as well as ring current intensification. However, an algorithm for such a purpose has not yet been developed. [4] In this letter we report a quantitative separation of the DD and UL components in the ionospheric electric field using the Natural Orthogonal Components (NOC) algorithm. As an example, we have studied a geomagnetic storm event on April 18, 2002, which is accompanied by a sawtooth event. The input data are AMIE electric potential in the high latitude ionosphere [Richmond et al., 1990]. Although there might be some uncertainty in the AMIE potential depending on the original dataset, our calculation results are very consistent with observations and previous studies.