Nonmigrating tidal coupling from the MLT region into the ionosphere and upper thermosphere
Final Report Abstract
Within the framework of CAWSES (Climate and Weather of the Sun-Earth System) contributions were made to the understanding that large-scale weather systems in the tropics do significantly influence the dynamics aloft. This discovery is an essential new realization and important for the “space weather” of the ionosphere-thermosphere-mesosphere (ITM) system. We proposed a scheme for the tropospheric tidal coupling into the ITM system. Latent heat release due to deep convective processes in the tropics (primarily thunderstorm cells) excites tidal waves propagating upward into the mesosphere, lower thermosphere (MLT) region reaching maximum amplitudes there. The predominance of wave-4 topography (separation of continents by about 90° in longitude) and land-sea weather differences in the tropics favours the excitation of the non-migrating DE3 tide, which is dominating by far the tidal spectrum in the MLT over large parts of the year. In this altitude region the presence of DE3 is observed in various quantities such as temperature, wind, or nitric oxide. When measurements are taken by satellites in a quasi-constant local time frame DE3 manifests itself as a longitudinal wave-4 pattern at low latitudes. The DE3 tidal wind modulation drives longitudinally varying currents in the ionospheric E-region. Related electric fields are mapped up along field lines and are the governing process behind the tidal coupling into the ionospheric F-region. The zonal electric fields modulate in turn the vertical plasma drifts that finally control the equatorial ionisation anomaly (EIA) longitudinal structure as has been found in 135.6 nm airglow brightness measurements. The low-latitude vertical plasma drift at 600 km altitude derived from ROCSAT-1 measurements also exhibits a prominent longitudinal wave-4 pattern, as does the current strength of the equatorial electrojet (EEJ) in the E-region. The longitudinal distribution of the wave-4 peaks is similarly present in location, local time, and seasons confirming a close connection between the vertical plasma drift and the equatorial electrojet. The crest-to-trough ratio of the EIA tracks closely the tidal variations of the vertical plasma drift but everything appears 1-2 hours later. This is consistent with the expected plasma transport time. Ionospheric signatures of DE3 tides are in general limited to daylight hours, supporting the fact that the coupling between neutrals and ions takes place primarily in the E-region, when the conductivity is sufficiently high. Nevertheless, it was shown that SE2 (also a contributor to wave-4) in the meridional wind contributes to the neutral-ion coupling at F-region heights. Thus the observed "wave-4" pattern in the ionosphere cannot be attributed solely to the dynamo process in the E-layer though the DE3 action there remains the leading contributor. In addition the DE3 tidal signal is coupled also directly into the upper thermospheric quantities like zonal wind, temperature or mass density. A further connection may exist, causing a feedback between the ionospheric plasma drift wave-4 pattern in the F-region and the neutral zonal wind at this altitude. Such a relation still has to be confirmed and thus remains with a question mark. The studies performed within this project have revealed explanations for prominent longitudinal structures in upper atmospheric quantities, which are not included in any of the atmospheric and ionospheric models.
Publications
- A statistical analysis of longitudinal dependences of upper thermospheric zonal winds at dip equator latitudes derived from CHAMP. J. Atmos. Solar-Terr. Phys., 69:1419–1430, 2007
K. Häusler, H. Lühr, S. Rentz, and W. Köhler
- Longitudinal variation of F region electron density and thermospheric zonal wind caused by atmospheric tides. Geophys. Res. Lett., 34, 2007
H. Lühr, K. Häusler, and C. Stolle
- The influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet. J. Geophys. Res., 113, 2008
H. Lühr, M. Rother, K. Häusler, P. Alken, and S. Maus
- Nonmigrating tidal signals in the upper thermospheric zonal wind at equatorial latitudes as observed by CHAMP. Ann. Geophys., 27:2643–2652, 2009
K. Häusler and H. Lühr
