[1] The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber-4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber-4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non-migrating tide mode DE3. We also found that the wavenumber-4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E-region and the zonal E Â B ion drifts in F-region. Our results support the suggestion that the longitudinal wavenumber-4 structure of the low latitude ionosphere should be originated from the non-migrating tide mode DE3.

[1] The 15-month climatology of medium-scale traveling ionospheric disturbances (MSTIDs) during a solar minimum period has been constructed from observations of a dense GPS receiver array in Central China. In total, 793 MSTID events are identified, with peaks in occurrence at 1500 LT and 0100 LT. The occurrence of MSTIDs decreases following an increase in geomagnetic activity, with 46% of the MSTIDS occurring in the daytime. Daytime MSTIDs are characterized by a major occurrence maximum around the winter solstice and by an equatorward propagation direction. The period, phase velocity, azimuth, and amplitude of daytime MSTIDs are 20-60 min, 100-400 m/s, 130°-270°, and 0.8-1.5%, respectively. The remaining 54% of the MSTIDs occurred at night, and were characterized by a peak in occurrence at the summer solstice and by a southwestward propagation direction. The period, phase velocity, azimuth, and amplitude of nighttime MSTIDs are 20-70 min, 50-230 m/s, 170°-300°, and 2-7%, respectively. The propagation directions and the seasonal behaviors support the view that daytime MSTIDs are an ionospheric manifestation of atmospheric gravity waves from the lower atmosphere, while a possible excitation mechanism of nighttime MSTIDs is the electrodynamics process caused by plasma instability in the F layer.

[1] We have investigated the propagation of large-scale traveling ionospheric disturbances (LSTIDs) during the super magnetic storm of 29-30 October 2003. Twodimensional total electron content (TEC) perturbation maps over North America were built using TEC data provided by the American GPS network and the International GNSS Service. Three LSTID events were observed in the range of 30°N-50°N, 60°W-110°W during this period. The first two LSTIDs occurred consecutively during 0620-0800 UT on 29 October at the local time of midnight, right after the onset of the big substorm; the third one was found at noon during the expansion phase of another substorm on 30 October. The phase fronts of these LSTIDs passed over the United States and traveled southwestward to the distance of $2000 km with the maximum front width of $4000 km and the duration of less than 2 hours. The maximum amplitude of TEC perturbations attained 3 total electron content units (TECUs). The results differ from the former observation of Afraimovich and Voeykov (2004) and Afraimovich et al. (2006), who reported a solitary LSTID propagating southwestward over the United States with the amplitudes of up to 14 TECU on 30 October 2003. We have checked the magnetic H component observed at the geomagnetic observatories in North America and found it is most likely that the auroral westward electrojet was the cause of the LSTIDs on 29 October. The source region for these TIDs was likely to be located several hundred kilometers north of 50°N. Cross-spectral analysis was conducted to obtain the global propagation characteristics of LSTIDs during this superstorm. Equatorward LSTIDs were found in all the three sectors of North America, Europe, and Asia, showing high correlation with the occurrence of auroral substorms.