Abstract:
Under the assumption that the diurnal range of the equatorial field represents the strength of the electrojet current, quasi-stationary periodicities in the intervals of 12 to 16 days and 5 to 7 days are inferred from the spectra of five sets of time series of the electrojet strength chosen from the period 1975-77. Many of these cyclic variations are also noticed in the spectra of the electrojet field at individual hours around local noon as well as in the corresponding series of noontime critical frequencies of F2 and Es from Kodaikanal, a station under the electrojet in south India. The periodicities in the interval of 5 to 7 days appear to arise from the modulation of ionospheric zonal wind by mixed Rossby-gravity waves. The longer-period variations seem to be associated with either the lunar semi-monthly tide or a harmonic of the solar synodic rotation period. A signal of a 4-day period is conspicuous by its persistence.
