Seasonal and Latitudinal Variations of the Electron Density Nonmigrating Tidal Spectrum in the Topside Ionospheric F-Region As Resolved from CHAMP Observations

Abstract:

In this study we present for the first time the nonmigrating tidal spectrum for the electron density in the topside ionosphere on global scale for different seasons at both solar maximum and minimum conditions. The electron density observations from the CHAMP (CHAllenging Minisatellite Payload) satellite provide evidence for prominent nonmigrating tides at different latitude regions. The diurnal tides DE1, D0, and DW2 appear preferably at middle and high latitudes during all the seasons. DE1 is only found in the northern middle and high latitudes, while D0 and DW2 are much stronger in the southern hemisphere. These tides are believed to be driven by the thermospheric winds through ion-neutral interactions. The semidiurnal tides SW1 and SW3 can also be found with appreciable amplitudes throughout the whole year at low and middle latitudes, but preferably during equinox seasons. At equatorial and low latitude regions the most prominent diurnal tides are DE2 and DE3. DE2 is found to be present at low latitude regions throughout the whole year with larger amplitudes in the southern hemisphere, while DE3 shows largest amplitudes (symmetric above the dip equator) at the EIA crest region during September solstice. A general anti-phase behavior between the EIA crest and trough is observed for the tides DE3, DE2, DW2 and SW3. We consider this as strong evidence for the modulation of the EIA electron density by the E-layer zonal electric field via the ion fountain effect. An exception makes the tidal component D0, which exhibits anti-phase variations between the two hemispheres. The phase value at the equatorial trough is half way between those of the two hemispheres. Presently we cannot give an explanation for it, and study concerning special model effort is further needed.