Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, V z in the interval 0047-0210 IST on the night of 23/24 December, 1991 ( A p = 14, K p - ). The fluctuations in F region vertical drift are found to be coherent with variations in B z (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H / X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations. Key words: Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)