PLANETARY WAVE AND SOLAR EMISSION SIGNATURES IN THE EQUATORIAL ELECTROJET

Recent analyses of observational data reveal the presence of perturbations in the E and F regions of the equatorial ionosphere with periods ranging from 2 to 45 days. The characteristic periods of many of these perturbations suggest an association with free Rossby (resonant mode) oscillations, perhaps excited either in the lower atmosphere or in situ. Zn the present work we analyze hourly magnetic observations from Huancayo Observatory, Peru (12.00 degrees S, 75.30 degrees W geographic; 0.72 degrees S, 4.78 degrees W geomagnetic), for the presence and persistence of these oscillations during the whole year of 1979. The measured variations can be interpreted in terms of oscillations of the wind field in the E region (approximately 100-160 km), which in turn cause perturbations in the electric fields generated by the wind-driven atmospheric dynamo and in the magnetic field intensity measured at the ground. The observations suggest that the effects of planetary wave oscillations with periods close to 2.5, 3, 6, 7, 9, 10.5, and 16 days may regularly propagate into the thermosphere and ionosphere, causing oscillations which are significant in magnitude. On the basis of an averaged periodogram analysis, we estimate that planetary wave effects may account for up to 75% of the total energy in Delta H values in the 2 to 35 day period range, suggesting that planetary waves may provide an important contribution to the dynamics and electrodynamics of the lower ionosphere and thermosphere. EUV fluxes during 1979 are noted to have a predominant 13.5-day periodicity during the first half of the year and the more typical 27-day oscillation during the latter half of 1979. These features can in principle affect the Delta H variations through their influence on the E region conductivity. We examine such influences here, especially those that affect the interpretation of the quasi 16-day oscillation.