Toward a better representation of the secular variation. Case study: The European network of geomagnetic observatories

Abstract

In the present paper we discuss a few issues regarding the secular variation (SV) and secular acceleration (SA) of the geomagnetic field that have consequences on mapping them at regional scales. Data from the European network of geomagnetic observatories have been analyzed from the perspective offered by existing long time series of annual means. The existence of high-frequency ingredients in the temporal change of the main field has been taken into account too. The importance of eliminating, from observatory and main field model data, prior to any discussion on secular variation, the signal related to external variations is demonstrated. Its consequences for SV analysis and/or mapping, including the jerk concept, are shown. Also, the importance of the geographical scale at which the SV is represented is discussed. To that aim, we used gufm1, IGRF and CM4 models for the main field from which the residual external signature was eliminated. The contribution of high-frequency ingredients to the map pattern is revealed. The results of the paper set additional observational constraints to the main field and geodynamo modeling.

Geomagnetic field declination: from decadal to centennial scales

“…We have chosen geographically close European observatories in order to enhance the visual effect, as they are affected by the same 22-and ∼ 80-year variations (see Demetrescu and Dobrica, 2014) and show similar time evolutions. The similarity of the 11-year signal in European observatories is also noticed in a previous paper (Dobrica et al, 2013).…”

“…These remaining external contributions are present mostly in the intensity elements of the recorded field (H , Z) and affect the declination less (e.g., Olsen and Mandea, 2007). The effects of external contributions in studying the secular variation have been emphasized and quantitatively shown for European observatories by Verbanac et al (2007), in terms of correcting annual means using information on external sources, by Wardinski and Holme (2011) in terms of a stochastic (covariant) modeling method and by Dobrica et al (2013) in terms of secular variation maps. Demetrescu and Dobrica (2014) tentatively show that the ∼ 80-year variation can be traced back to the 15th century using three long time series of declination for London (Malin and Bullard, 1981), Rome (Cafarella et al, 1992a, b), and Munich (Korte et al, 2009).…”

Geomagnetic field declination: from decadal to centennial scales

“…We have chosen geographically close European observatories in order to enhance the visual effect, as they are affected by the same 22-and ~80-year variations (see Demetrescu and Dobrica, 2014) and show similar time evolutions. The similarity of the 11-year signal in European observatories is also noticed in a previous paper (Dobrica et al, 2013). get rid of variations related to the first harmonic of the 11-year variation, attenuates the sharp variation of the declination time derivative seen in the raw data (upper plot).…”

“…These remaining external contributions are present mostly in the intensity elements of the recorded field (H, Z), and affect less the declination (e. g., Olsen and Mandea, 5 2007). The effects of external contributions in studying the secular variation have been emphasized and quantitatively shown for the European observatories by Verbanac et al (2007), in terms of correcting annual means using information on external sources, by Wardinski and Holme (2011), in terms of a stochastic (covariant) modeling method, and by Dobrica et al (2013), in terms of secular variation maps. Recent studies by Gillet et al (2010Gillet et al ( , 2015 and Holme and de Viron (2013) point to a possible ~6 year variation originating in the core.…”

Core surface sub-centennial magnetic flux patches: characteristics and evolution

“…Recently, Metman et al (2017) showed that the reversed flux patches are responsible for two-thirds of the dipole decay over the twentieth century the rest of the decay being caused by the evolution of the normal field. Secular variation studies developed by Dobrica (2005, 2014) and Dobrica et al (2013) on long time series (100-150 years) of observatory data showed that the surface field and its SV are constituted of oscillations at time scales of ~ 11, ~ 22 and ~ 80 years, which are superimposed on a (quasi)linear trend denoted steady variation. The "decadal, " 11-year oscillation is considered as solar-cycle-related, contaminating the observatory annual means, while the ~ 22, ~ 80 year, and steady constituents are of internal origin.…”