Intra‐annual variability of the low‐latitude ionosphere due to nonmigrating tides

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 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 ( SW 4), and the eastward‐propagating diurnal tides with s = −3 ( DE 3) and s = −2 ( DE 2), respectively. DE 3 is the dominating tide during the other months. The results presented indicate that nonmigrating tides other than DE 3 may significantly impact the longitudinal structure of the low‐latitude ionosphere during certain months.