VHF Radar Observations of Plasma Irregularities in the Counter Electrojet

VHF radar observations made from Thumba, India during the period 1972-73, on counter electrojet days, showed the presence of weak plasma irregularities even after the reversal of the normal electrojet currents. A time evolution study of the generation of these irregularities shows that though the irregularities responsible for the radar echoes vanish prior to the onset of counter electrojet, on days when the reversed currents are strong enough and continue to flow for a relatively longer duration, the irregularities, though weak, reappeared during the counter electrojet period. The generation of these irregularities by the cross-field instability mechanism, necessitates the existance of downward electron density gradient in the region of counter electrojet. Another possible explanation could be that, the counter electrojet is only partial and the radar echoes during this period are due to plasma irregularities that exist in the region of normal electrojet flow. From a detailed study of the time evolution of the radar echoes, an attempt is made to resolve this problem.