[1] The daily averaged Solar EUV Monitor (SEM)/Solar Heliospheric Observatory (SOHO) EUV measurements, solar proxies, and foF2 data at 20 ionosonde stations in the east Asia/Australia sector are collected to investigate the solar activity dependences of the ionospheric peak electron density (NmF2). The intensities of solar EUV from the SEM/ SOHO measurements from 1996 to 2005 show a nonlinear relationship with F107, and the SEM/SOHO EUV can be better represented by a solar activity factor P = (F107 + F107A)/2. Seasonal and latitudinal dependences are found in the solar activity variation of NmF2 in the east Asia/Australian sector. The slope of NmF2 with P in the linear segment further shows similar annual variations as the background electron densities at moderate solar activity. Observations show a nonlinear dependence of NmF2 on solar EUV (the saturation effect of NmF2 for high solar EUV). On the basis of a simple model of photochemistry, taking the neutral atmospheric consequences into account, calculations at fixed height simulate the saturation effect of NmF2, but the observed change rate of NmF2 with P is inadequately reproduced. Calculations taking into account the influence of dynamics (via a simple model of the solar EUV dependence of the ionospheric height) tend to reproduce the observed change rate of NmF2. Results indicate that besides solar EUV changes, the influence of dynamics and the atmospheric consequences should substantially contribute to the solar activity variations of NmF2.

[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] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.