OBSERVATIONS OF PC5 MICROPULSATION-RELATED ELECTRIC-FIELD OSCILLATIONS IN THE EQUATORIAL IONOSPHERE

A 54.95-MHz coherent backscatter radar, an ionosonde and the magnetome ter located at Trivandrum in India (8.5-degrees-N, 77-degrees-E, 0.5-d egrees-N dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation e vent, which occurred during an intense magnetic storm on 24 March 1991 (A(p) = 161). Simultaneous 100-nT-level fluctuations are also observe d in the H-component at Brorfelde, Denmark (55.6-degrees-N gm) and at Narsarsuaq, Greenland (70.6-degrees-N gm). Our study of the above obse rvations shows that the E-W electric field fluctuations in the E- and F-regions and the magnetic field fluctuations at Thumba are dominated by a near-sinusoidal oscillation of 10 min during 1730-1900 IST (1200- 1330 UT), the amplitude of the electric field oscillation in the equat orial electrojet (EEJ) is 0.1-0.25 mV m-1 and it increases with height , while it is about 1.0 mV m-1 in the F-region, the ground-level H-com ponent oscillation can be accounted for by the ionospheric current osc illation generated by the observed electric field oscillation in the E EJ and the H-component oscillations at Trivandrum and Brorfelde are in phase with each other. The observations are interpreted in terms of a compressional cavity mode resonance in the inner magnetosphere and th e associated ionospheric electric field penetrating from high latitude s to the magnetic equator.