Variability in the longitudinal structure of the low-latitude ionosphere

In-situ plasma density measurements from the CHAMP satellite between about 350-420 km altitude are used to delineate intra-annual variations in the longitudinal structure of the low-latitude F-region ionosphere. It is shown that the longitude structures during mid-day local times are dominated by the space-based longitudinal wavenumbers k_s=2, 4 and 3 in January, July, and December, respectively. These conform to the same dominating k_s-values characterizing solar thermal tide zonal winds in the dynamo region, namely the westward-propagating semidiurnal tide with planetary-fixed zonal wavenumber s = 4 (SW4), and the eastward-propagating diurnal tides with s = -3 (DE3) and s = -2 (DE2), respectively. DE3 is the dominating tide during the other months. The results presented demonstrate that the dominate longitudinal structure of the low-latitude ionosphere exhibits month-to-month variability in a manner that is consistent with nonmigrating tides in the E-region and that nonmigrating tides other than DE3 have an important role in the development of longitudinal structures in the low-latitude ionosphere during certain months.