VARIATION OF TYPE-I PLASMA-WAVE PHASE-VELOCITY WITH ELECTRON-DRIFT VELOCITY IN THE EQUATORIAL ELECTROJET

Using a 54.95-MHz coherent backscatter radar at Thumba (8.5-degrees-N, 77-degrees-E; 0.5-degrees-N dip angle), the phase velocity variations of type I plasma waves generated by the modified two-stream instabili ty process in the equatorial electrojet have been measured on several occasions of large and rapid electric field fluctuations associated wi th geomagnetic substorms and storms. The measurements show a clear lin ear variation of type I wave phase velocity (V(p)I) over a considerabl e range with the simultaneously measured phase velocity (V(p)II) of gr adient drift instability-generated type II plasma waves. Since V(p)II is proportional to the drift velocity of electrons, the observed varia tions of V(p)I with V(p)II mean that V(p)I varies in step with the var iations of the electron drift velocity V(ey). The observed variation o f V(p)I with V(ey) is consistent with the current theoretical understa nding that the plasma wave-associated electric fields cause anomalous electron diffusion and heating which result in the stabilization of th e type I wave phase velocity near the increased value of the ion-acous tic velocity. The enhanced electron temperatures estimated from the la rgest values of the observed V(p)I are found to be 2.0-2.4 times the a mbient value in the absence of waves.