Detailed analysis of the scalar magnetic field data from Ørsted satellite for quiet days from April 1999 to March 2000 has been undertaken to study the equatorial electrojet (EEJ) phenomenon. An objective technique has been adopted for the identification of the EEJ from the satellite data and estimation of the standard parameters associated with it. EEJ strength computed using the satellite data and simultaneous ground magnetic observatory data, for the Indian and American sectors, correlate very well authenticating the method used. Estimated zonal variation in the EEJ parameters such as peak current intensity (J0), and total current (I+) are broadly consistent with the earlier observations. We, however, observe that the width of the EEJ varies considerably with longitude, a feature not seen in the Pogo data. The study shows that the EEJ axis (center of EEJ) closely follows the dip equator at altitude of 106 km, but there is a small departure that undergoes diurnal variation, with a minimum at noon. The globally averaged EEJ amplitude follows the expected diurnal pattern. Principal component analysis technique reveals that first four components can explain around two thirds of the electrojet variability. The first component, which contributes a little over 30% to the observed variance, could be identified with the global variation of the EEJ emanating from the day‐to‐day variability of the migrating tides. The second and fourth components, which account for around 15 and 10% of the variance, respectively, are driven by forcing that depends on whether the location of the EEJ in that sector is in the Northern or Southern Hemisphere. The third component provides maximum contributions wherever the geomagnetic and dip equators are sufficiently close, accounting for 12.5% of the variance. The remaining components could be associated with contribution of nonmigratory tides or other unknown mechanisms. Thus the present study suggests that besides conductivity, atmospheric tidal modes play important role in defining the zonal variability of the EEJ current system.
Abstract.Ionospheric modulation of compressible hydromagnetic waves has been examined using the scalar magnetic field data obtained from the Oersted satellite and ground magnetic data from the Japanese sector. It is shown that the day side ionosphere introduces significant phase delays for frequencies in excess of about 20 mHz. The phase delay increases with frequency and with increase in the conductivity of the ionosphere. There is also an increase in the amplitude of the wave above equatorial day side ionosphere. The uniformly excellent coherence between the satellite and ground oscillations and systematically good correlation between the filtered time series at the two levels suggests that the compressible hydromagnetic waves are phase coherent over a wide range of latitudes.
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