| Popis: |
Winds in the upper atmosphere vary with time, location and geomagnetic activity among other causes. Understanding these changes is very important, because they affect the circulation and the behavior of the ionosphere, the ionized portion of the upper atmosphere. The ionosphere, in turn, affects technologies such as GPS and HF communication. The Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission provided dawn and dusk zonal neutral winds at an altitude of ∼260 km. GOCE measurements from low and mid-latitudes will be used to study the variability of thermospheric zonal winds. The spatial and temporal structures in GOCE zonal winds were studied during geomagnetically quiet periods. Correlations between different points in the globe were computed during December solstice 2009 to 2012 and the year-to-year variability was investigated. December solstice 2009 and June solstice 2010 were also compared. Periodic structures in longitude were identified. The effect of atmospheric tides in the GOCE zonal winds was also studied for these periods and the structures in longitude that they produce were analyzed. A year-to-year progression was recognized, for both dawn and dusk. The structures present were compared to model results from the Climatological Tidal Model of the Thermosphere (CTMT). The CTMT results show good agreement during dawn, and the variations produced by individual tidal components were studied using the model. Finally, the evolution in time of the response of the GOCE zonal winds was investigated. The mean response was calculated using the variations obtained for 19 geomagnetic storms. During dawn, the perturbations are eastward for mid-higher geomagnetic latitudes in both hemispheres. For mid to low geomagnetic latitudes, westward variations are observed. During dusk, the perturbations for all latitudes are westward. It was determined that during dawn (dusk), the response in the GOCE zonal winds reaches the geomagnetic equator 12 (18) hours after the storm onset. These effects last until ∼27 hours. |
