Wave-driven variability in the ionosphere-thermosphere-mesosphere system from TIMED observations: What contributes to the "wave 4"?

Numerous observations and model studies made during the past 5 years have unequivocally revealed that the ionosphere and thermosphere owe a considerable amount of their longitudinal, local time, seasonal latitudinal and day-to-day variability to waves originating in the lower part of the atmosphere. The most prominent pattern is the four-peaked ("wave 4") longitudinal structure frequently observed by (quasi-) Sun-synchronous satellites in a variety of ionospheric and thermospheric parameters. The "wave 4" has often been attributed to the diurnal, eastward, wave number 3 (DE3), nonmigrating tide alone. A more detailed analysis of TIMED observations, supported by physics-based empirical modeling and data from the CHAMP satellite, now indicates that this interpretation needs to be revised. Secondary wave generation due the nonlinear interaction between the migrating diurnal tide and the DE3 leads to a large stationary planetary wave 4 (SPW4) and a large semidiurnal, eastward wave number 2 (SE2) tide in the equatorial zonal wind at E-region heights. Combined amplitudes can equal those of the DE3. SE2 penetrates into the upper thermosphere with transequatorial wind speeds in excess of 10 m/s. This paper discusses the resulting implications for electric field generation in the E-region and tidal-ionosphere coupling in the F-region and provides observational constraints for future modeling efforts.