Repeat station data compared to a global geomagnetic field model

Vincent Lesur Vincent Lesur,Monika Korte Monika Korte

doi:10.4401/ag-5410

Vincent Lesur Vincent Lesur, Monika Korte Monika Korte

Open Access

https://doi.org/10.4401/ag-5410

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Abstract

<p>Geomagnetic repeat station surveys with local variometers for improved data reductions have been carried out in Germany for about ten years. For nearly the same time interval the satellites Ørsted and CHAMP have provided a good magnetic field data coverage of the whole globe. Recent global field models based on these satellite data together with geomagnetic observatory data provide an improved description of the core field and secular variation. We use the latest version of the GFZ Reference Internal Magnetic Model to compare the magnetic field evolution predicted by that model between 2001 and 2010 to the independent repeat station data collected over the same time interval in Germany. Estimates of crustal bias at the repeat station locations are obtained as averages of the residuals, and the scatter or trend around each average provides information about influences in the data from field sources not (fully) described by the global model. We find that external magnetic field signal in the order of several nT, including long-term trends, remains both in processed annual mean and quiet night time repeat station data. We conclude that the geomagnetic core field secular variation in this area is described to high accuracy (better than 1 nT/yr) by the global model. Weak long-term trends in the residuals between repeat station data and the model might indicate induced lithospheric anomalies, but more data are necessary for a robust analysis of such signals characterized by very unfavorable signal-to-noise ratio.</p>

Highlights

Geomagnetic repeat station surveys are carried out in many countries for the purpose of regionally mapping the Earth's magnetic field components and their secular variation
We have compared the most recent GRIMM magnetic field model with German repeat station data in order to investigate the accuracy of the data and the model and to evaluate the benefits of repeat station data in areas comparatively densely covered by magnetic observatories
The subtraction of GRIMM3 core field predictions from the data has shown that significant external field contamination is present both in repeat station annual means and quiet night time data

Summary

Introduction

Geomagnetic repeat station surveys are carried out in many countries for the purpose of regionally mapping the Earth's magnetic field components and their secular variation. VCQN and the resulting CQN are taken as averages over a range of night time values, in general in the order of a few hours, when the magnetic K index at Niemegk observatory is lowest and the variation recordings visually appear best to reflect a nearly constant quiet internal field level These quiet night time results for different days (nights) within the year are further reduced to common annual means centered on the middle of the year by CAM (x,tAM) = OCAM (x,tAM) + [CQN (x,tQN) − OCQN (O,tQN)]. Repeat station data reduced by Equation 2 will be affected This remaining annually averaged external variation signal is quite homogeneous over regions the size of Germany and can reasonably be estimated empirically from the three or four (BFO only operates since 2004) German observatories [Korte and Thébault 2007].

The GRIMM-3 model and the German vector magnetic anomaly model

Secular variation comparison and crustal biases

Findings

Discussion and conclusions