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

V Dobrica,C Stefan,C Demetrescu

doi:10.5047/eps.2012.12.001

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.

Highlights

An inspection of sources that contribute the observed geomagnetic field at Earth’s surface reveals the well known three main contributors, namely the core field, the lithospheric field and the external field (Mandea and Purucker, 2005; Olsen et al, 2007)
Looking at data provided by main field models based on observatory data from the same angle we did in our discussion in the previous sections, reveals the presence of a reminiscent 11-year signal in the time series provided by the model
We confirmed the well known presence in the annual means of geomagnetic elements of a 11-year solar-cyclerelated (SC) signal due to incomplete averaging out external effects and their induced counterparts, modulated by solar activity (Chapman and Bartels, 1940; Yukutake, 1965; Bhargava and Yacob, 1969; Alldredge, 1975, 1976; Courtillot and Le Mouel, 1976; Alldredge et al, 1979; Yukutake and Cain, 1979; Demetrescu et al, 1988; Verbanac et al, 2007; Wardinski and Holme, 2011) and quantitatively showed that the time change of the signal is of the same order of magnitude as that of the internal ingredients of the measured field

Summary

Introduction

An inspection of sources that contribute the observed geomagnetic field at Earth’s surface reveals the well known three main contributors, namely the core field ( called the main field), the lithospheric field and the external field (Mandea and Purucker, 2005; Olsen et al, 2007). In the present paper we discuss, on data from the European network of geomagnetic observatories, a few issues regarding the secular variation and secular acceleration, with consequences on mapping them at regional scales, from the perspective of the existence of high-frequency ingredients (Demetrescu and Dobrica, 2005, 2013) in the temporal change of the main field. We can fit a second order polynomial to each segment of the time series defined according to the curvature sign, in the time interval covered with a homogeneous data set from as many European observatories as possible This will allow describing the evolution of the internal field and of its time change in a suitable way for map representation, as: Eint (t ) = a0 + a1 t + a2( t ). The high noise level, which will be discussed

The Time Perspective Offered by Available Long Time-Series of Data

On the Leakage of External Signal into Main

A Discussion on the Secular Variation Evolution in the 20th Century in Europe

Conclusions