Ionosonde data of Kodaikanal (Geog.Long. 77¿ 29'E, dip 3.0¿N) and Huancayo (Geog.Long. 75¿18'W, dip 2.0¿N) are used to show the simultaneous occurrence of a transient disturbance in F region height of composite polarity in day and night sectors near the dip equator during the auroral substorm activity on 20 August 1979. At Kodaikanal which is on the nightside at the time of the substorm activity, h'F first underwent an abrupt and rapid decrease (80 km in 1 hr) followed by a much larger increase (120 km in 1 hr). Perturbation in hpF2 of exactly opposite polarity was simultaneously seen at Huancayo which is on the dayside. The decrease in h'F at Kodaikanal (increase in hpF2 at Huancayo) occurred in association with an increase in polar cap potential drop, ϕ (estimated from IMF parameters), and the subsequent increase (decrease at Huancayo) with a decrease in polar cap potential. The F-region height disturbance is interpreted as the manifestation of a global transient composite disturbance in equatorial zonal electric field caused by the prompt penetration of substorm-related high latitude electric fields into the equatorial ionosphere. The polarity pattern of the electric field distrubance is consistent with the global convection models which predict westward (eastward) electric fields at night (by day) near the geomagnetic equator in response to an increase in polar cap potential drop, and fields of opposite signs for a decrease in polar cap potential. ¿ American Geophysical Union 1992