Using a 54.95-MHz coherent backscatter radar at Thumba (8.5¿N, 77¿E; 0.5¿N dip angle), the phase velocity variations of type I plasma waves generated by the modified two-stream instability process in the equatorial electrojet have been measured on several occasions of large and rapid electric field fluctuations associated with geomagnetic substorms and storms. The measurements show a clear linear variation of type I wave phase velocity (VpI) over a considerable range with the simultaneously measured phase velocity (VpII) of gradient drift instability-generated type II plasma waves. Since VpII is proportional to the drift velocity of electrons, the observed variations of VpI with VpII mean that VpI varies in step with the variations of the electron drift velocity Vey . The observed variation of VpI with Vey is consistent with the current theoretical understanding that the plasma wave-associated electric fields cause anomalous electron diffusion and heating which result in the stabilization of the type I wave phase velocity near the increased value of the ion-acoustic velocity. The enhanced electron temperatures estimated from the largest values of the observed VpI are found to be 2.0--2.4 times the ambient value in the absence of waves. ¿ American Geophysical Union 1993