The Role of Geomagnetic Observatory Data during the Swarm Mission
Ridley, Victoria; Macmillan, Susan; Hamilton, Brian
British Geological Survey, UNITED KINGDOM

The ESA Swarm mission to identify and measure very accurately the different magnetic signals that arise in the Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere, which together form the magnetic field around the Earth, has increased interest in magnetic data collected on the surface of the Earth at observatories. The scientific use of Swarm data and Swarm-derived products is greatly enhanced by combination with observatory data and indices. The strength of observatory data is that they are very stable over long periods of time with great care being taken with temperature control and correction, platform stability and magnetic cleanliness at each site. As part of the Swarm Level-2 data activities, plans are in place to distribute such ground-based data along with the Swarm data as auxiliary data products. We describe here the preparation of the data set of ground observatory hourly mean values, including procedures to check and select observatory data spanning the modern magnetic survey satellite era. Existing collaborations, such as INTERMAGNET and the World Data Centres for Geomagnetism, are proving invaluable for this.

In addition, we discuss other possible combined uses of satellite and observatory data. Quasi-definitive 1-second data are now available from a number of observatories worldwide. A preliminary study using Champ satellite data has shown that these observatory data could prove beneficial as a secondary tool for Swarm magnetic data validation; however, to avoid ionospheric and magnetospheric interference requires satellite-observatory crossings to occur during quiet, local night time. Whether or not such passes will transpire during the period of Swarm Calibration/Validation is highly dependent on the initial timing of orbital insertion and how active the magnetic field is. Alternatively, we consider exploiting 1-minute data, collected from a much larger global network of observatories. Removing an estimate of the main field from both the observatory and satellite observations during crossings could provide a baseline for detecting abnormalities and aid with Swarm measurement validation, whilst differences observed with solar zenith angle may constrain thermal fluctuations not otherwise accounted for. It is possible that early validation results with magnetic data from the Swarm mission will be presented if launch is successful in the preceding months.