The low-latitude electric fields are generally modeled by numerically solving a two-dimensional potential equation developed from assumptions of equipotential field lines and current continuity. Additional assumptions are used to derive simple formulas for the vertical electric field EL and zonal electric field E&phgr; in the plane of the magnetic dip equator. Electric fields obtained from the simple model will be compared against the electric fields of the more complete numerical model to demonstrate the validity of the simple model solution and to identify the range of validity. The simple electric field model is used to directly identify the primary causes of the low-latitude electric field structure. The observed structure has sources that can be divided into components: (1) polarization fields created by divergences in neutral-wind dynamo currents and (2) curl-free effects demanded by ∇¿E=0. It is shown that the evening prereversal enhancement of E&phgr; is caused by curl-freeness demands of the rapidly changing EL near sunset. This supports cursory statements of Rishbeth [1971> on the cause of the postsunset enhancement. Finally, the resulting altitude variation of the zonal electric field depends on the type of source. For polarization field sources the altitude variation is approximately proportional to L2. For curl-free sources the altitude variation is related to the rate of change of EL with respect to longitude. ¿ 1998 American Geophysical Union