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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 35, issue 4 | Copyright

Special issue: The Earth’s magnetic field: measurements, data, and applications...

Ann. Geophys., 35, 939-952, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 17 Aug 2017

Regular paper | 17 Aug 2017

Estimating error statistics for Chambon-la-Forêt observatory definitive data

Vincent Lesur1, Benoît Heumez1, Abdelkader Telali1, Xavier Lalanne1, and Anatoly Soloviev2,3 Vincent Lesur et al.
  • 1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR 7154 CNRS, 75005 Paris, France
  • 2Geophysical Center of the Russian Academy of Sciences (GC RAS), Molodezhnaya Str. 3, 119296 Moscow, Russia
  • 3Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (IPE RAS), Bolshaya Gruzinskaya Str. 10-1, 123242 Moscow, Russia

Abstract. We propose a new algorithm for calibrating definitive observatory data with the goal of providing users with estimates of the data error standard deviations (SDs). The algorithm has been implemented and tested using Chambon-la-Forêt observatory (CLF) data. The calibration process uses all available data. It is set as a large, weakly non-linear, inverse problem that ultimately provides estimates of baseline values in three orthogonal directions, together with their expected standard deviations. For this inverse problem, absolute data error statistics are estimated from two series of absolute measurements made within a day. Similarly, variometer data error statistics are derived by comparing variometer data time series between different pairs of instruments over few years. The comparisons of these time series led us to use an autoregressive process of order 1 (AR1 process) as a prior for the baselines. Therefore the obtained baselines do not vary smoothly in time. They have relatively small SDs, well below 300pT when absolute data are recorded twice a week – i.e. within the daily to weekly measures recommended by INTERMAGNET. The algorithm was tested against the process traditionally used to derive baselines at CLF observatory, suggesting that statistics are less favourable when this latter process is used. Finally, two sets of definitive data were calibrated using the new algorithm. Their comparison shows that the definitive data SDs are less than 400pT and may be slightly overestimated by our process: an indication that more work is required to have proper estimates of absolute data error statistics. For magnetic field modelling, the results show that even on isolated sites like CLF observatory, there are very localised signals over a large span of temporal frequencies that can be as large as 1nT. The SDs reported here encompass signals of a few hundred metres and less than a day wavelengths.

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Short summary
Chambon-la-Forêt magnetic observatory distributes definitive second data that are contaminated by noise with a standard variation below 400 pT. This noise is low compared to the international standard set by INTERMAGNET. It is mainly due to local signals of unknown origins that have short wavelengths in space (less than 200 m) and in time (less than a day).
Chambon-la-Forêt magnetic observatory distributes definitive second data that are contaminated...