Une méthode d'observatoires virtuels pour décrire les variations temporelles du champ géomagnétique et applications aux mesures de la mission Swarm

摘要

A description of the temporal variations of the main geomagnetic field (i.e., the secular varia-tion or SV) is crucial to the understanding of core dynamo generation. It is known with highaccuracy at observatory locations, which are globally unevenly located, hampering the determi-nation of a global pattern of these variations. Satellites have allowed global surveys of the fieldand its SV. Their data has been used by global spherical harmonic models using data selectioncriteria to reduce external contributions. SV small spatial scales may not be well described bythese models and can show significant errors compared to ground measurements. This studyattempts to extract temporal variation time series from satellite measurements as it is doneat observatory locations. We follow a Virtual Observatories (VO) approach, defining a globalmesh of VOs at satellite altitude. We apply an Equivalent Source Dipole (ESD) technique. Foreach VO and a given time interval all measurements are reduced to a unique location, leading totime series similar to those available at the ground. Synthetic data is first used to validate theapproach. We then apply our scheme to Swarm mission measurements and locally compare theVO-ESD derived time series to ground observations and to satellite-based model predictions.The approach is able to describe field’s time variations at local scales. The global mesh of VOtime series is used to derive global spherical harmonic models. For a simple parametrizationthe model well describes the trend of the magnetic field both at satellite altitude and at thesurface. Nevertheless more complex modelling can be made to properly profit of VO-ESD timeseries.