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.

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Abstract. Observations of mesospheric winds over a period of four years with the partial re¯ection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70±98 km. The meridional winds exhibit overall transequatorial¯ow associated with dierential heating in the Northern and Southern Hemispheres. At lower altitudes (70±80 km) the mean zonal winds reveal easterly¯ow during summer and westerly¯ow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80±98 km) and at all altitudes during equinox periods, the mean zonal¯ow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at speci®c harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.

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Abstract. It has been shown that the failure of the classical WKB approach in reproducing the correct frequency spectrum and spatial structures of field line resonances should be attributed to the inabihty of the method to provide a natural solution close to the turning points. A direct analytic solution has been formulated to derive the toroidM field hue resonance structure. Finally, the solutions thus obtained have been compared with the numerically found exact solutions in order to test the authenticity of the formalism.

It is shown that the asymptotic evolution of a finite-amplitude Alfvén wave propagating parallel to the uniform magnetic field in a warm homogeneous collisionless plasma is governed by the modified nonlinear Schrödinger equation. The dispersion is provided by the ion finite Larmor radius (FLR) effects in the momentum equation and the Hall current and electron pressure corrections to the generalized Ohm's law. In the cold plasma limit the equations reduce to those available in the literature. It is suggested that these calculations can have a bearing on the investigation of the structure of MHD waves in the solar wind.