Relative secular variations of the Earth’s magnetic field in the years 1966–2016 along the profile across the main tectonic units of Poland (Zgorzelec-Wiżajny profile)

Anna Wojas,Teresa Grabowska

doi:10.7494/geol.2019.45.3.183

Abstract

The paper summarizes 50 years of research on relative local secular variations of the total magnetic intensity (TMI) along Zgorzelec-Wizajny profile (Z-W profile) through the years 1966–2016. The Z-W profile (approximately 650 km length) cuts through Europe’s major tectonic units, including a zone of Variscan folds in the Paleozoic platform (PP), the Trans-European Suture Zone (TESZ) and the Polish part of the East European Craton (EEC). Measurements of TMI were made at 31 sites of the profile and reduced to the base recordings at the Central Geophysical Observatory in Belsk. Based on linear approximations of relative geomagnetic field values changes in time, relative annual changes in time (relative secular variations) expressed in nanoteslas [nT] per year were calculated.The study on relative geomagnetic field values revealed slower secular variations of the geomagnetic field in EEC as compared to those in PP and TESZ. However, they show lower amplitudes of these variations. An interesting phenomenon are rapid changes in the time of the relative values (trends) of TMI starting from the year 2000, also expressed as the relative secular variations per year, particularly visible in PP area. In the qualitative sense, relative secular variations show a correlation with the main geological units in the Polish territory and also important elements in the seismic and geothermal model of the lithosphere.

Highlights

The geomagnetic field changes in time and space
The calculated relative values of ∆Tn,t are expressed by the formula: A method to determine the relative local changes of the geomagnetic field, as described in the work by Małoszewski & Jankowski (1997), and modified (Bojdys et al 2001) was applied
Another important feature of the geomagnetic field changes over time observed along the Z-W profile, is the visible reduction of its ΔTn,t relative values and the appearance of different trends of their changes in time from approximately the year 2000 (Fig. 4)

Summary

Introduction

The geomagnetic field changes in time and space. Long-term changes of the geomagnetic field, commonly referred to as geomagnetic secular variations (SV), are typically shown on a global scale in the form of secular variation maps. Geomagnetic secular variations are caused by magnetohydrodynamic processes taking place in the Earth’s outer core and in a transition zone of the outer core and the mantle itself. The origin of geomagnetic secular variations is connected with the generation of the main magnetic field of our planet. In the 1930s, the first research conducted on a local scale and based on long-term changes of the geomagnetic field (i.e., the secular anomalies of the Earth’s magnetic field changes) was initiated (Yanovskiy 1978).

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