On large poleward‐directed electric fields at sub‐auroral latitudes

Intense poleward‐directed electric fields were observed by OGO‐6 in the pre‐magnetic midnight F‐region, close to the Harang discontinuity and at latitudes of less than 60° invariant. These results support the recent measurements of Smiddy et al. (1977). Both sets of data are consistent with a compression of equipotentials on the low latitude edge of the pre‐midnight convection cell, caused by an impediment of the sub‐storm‐related southwestward expansion of the convection pattern.