Thermospheric densities for the Swarm satellite mission

The European Space Agency (ESA) Swarm mission was launched in November 2013 and consists of three identical satellites flying in near-polar orbits. One satellite is flying at about 515 km, while the other two satellites are flying side-by-side at lower altitudes, starting at 480 km altitude and slowly descending due to atmospheric drag to their current 445 km altitude. This coverage of altitudes, together with the satellite payload that includes an accelerometer and GPS receiver, makes the mission particularly suited for atmospheric density retrieval. Unfortunately, the Swarm accelerometers suffer from several anomalies which limits their usefulness for density retrieval. Currently, only accelerometer observations from one of the lower flying satellites (Swarm-C) can be used to generate high-resolution thermospheric densities. However, all satellites deliver high-quality GPS data and an alternative processing strategy has been developed to derive thermospheric densities from these observations as well.

This presentation describes the processing strategy that is used to derive thermospheric densities from the Swarm accelerometer and GPS observations and presents the latest results. The relatively smooth GPS-derived densities have a temporal resolution of about 20 minutes, and show variations due to solar and geomagnetic activity, as well as seasonal, latitudinal and diurnal variation. For analysis of higher-resolution phenomena, only the accelerometer-derived densities can be used. All Swarm thermospheric densities are available for users at the dedicated ESA Swarm website (ftp://swarm-diss.eo.esa.int), as well as at our thermospheric density database (http://thermosphere.tudelft.nl). This database also includes thermospheric densities for the CHAMP, GRACE and GOCE satellites. For future work, it is planned to further improve the Swarm densities, especially for low solar activity conditions, by including a more sophisticated radiation pressure modelling of the Swarm satellites. In addition, it is planned to extend our database with thermospheric densities for the GRACE-FO mission.