Numerical techniques are used to compare two‐dimensional and three‐dimensional analyses of the spreading of ionospheric electric fields. Specific cases are considered in which a source electric field is applied at 150‐km altitude and within a limited horizontal distance L. It is found that horizontal spreading of fields at ionospheric heights can be accurately calculated by using a two‐dimensional model with height‐integrated conductivities, provided that L is 10 km or greater. For L < 10 km a full three‐dimensional model must be used. Three‐dimensional calculations must also be used to relate electric fields measured at balloon altitudes (˜30 km) to overhead ionospheric fields if L < 100 km. Calculated ionospheric fields depend strongly on horizontal conductivity gradients, fields spreading more easily in the direction of decreasing conductivity.
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C. G. Park (1979) studied this question.
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