Geoinformation Issues Vol. 10, No 1 (10), 17–26/2018 A study of changes in the Earth’s magnetic field in Poland in the years 1954–2017; reducing the archival values of magnetic declination at the points of the magnetic network to a uniform epoch Elzbieta Welker Institute of Geodesy and Cartography, 27 Modzelewskiego St., 02-679, Warsaw, Poland Tel.: +48 22 3291913, Fax: +48 22 3291950, E-mail: email: [email protected] ORCID: https://orcid.org/0000-0003-4373-367X Anna Jedrzejewska Institute of Geodesy and Cartography, 27 Modzelewskiego St., 02-679, Warsaw, Poland Tel.: +48 22 3291913, Fax: +48 22 3291950, E-mail: email: [email protected] ORCID: https://orcid.org/0000-0003-2887-5602 Abstract: At the turn of the 1980s and 1990s, the Institute of Geodesy and Cartography developed a method of reducing magnetic declination based on 40-year long magnetic observations at repeat stations. This method assumed that changes in the magnetic field at repeat stations can be described using the polynomial of the 3rd degree as a function of the position of the point (φ, λ) and the measurement epoch. The Earth’s magnetic field as a temporal-spatial physical phenomenon of a variable, unpredictable nature is, however, difficult to model in a longer period of time. At the turn of the centuries the magnetic declination values at the points of the magnetic network were reduced to the 2000.5 epoch using the polynomial function for field modelling, based on the analysis of the actual changes in magnetic declination at repeat stations and in magnetic observatories. The unified set of points of the magnetic network with declination reduced to the 2000.5 epoch constituted the base for further studies of geomagnetic field. In the last dozen or so years, the network of the magnetic points has been updated by the new measure- ments of magnetic declination at over 300 points. Not all previous locations were suitable for re-measurements. In this case, a point in the nearby region that met the conditions for magnetic measurements was selected. New points, with the values of magnetic declination reduced to the measurement epoch, updated and expanded the data bank. An analysis of the course of actual changes in the Earth’s magnetic field in the observatories of Central Europe and at the repeat stations of the national magnetic control of Poland from the measurement epoch, i.e. from the 1950s to the current epoch, made it possible to unify the set with points with the magnetic declination determined. From now all the points will have the value of declination assigned to the 2017.5 epoch. Thus, the procedure for the reduction of magnetic data to subsequent epochs will be simplified. The decision about the possible re-updating of the data will be taken after analysing the course of further changes in the Earth’s magnetic field in Central Europe from the 2017.5 epoch. Keywords: Earth’s magnetic field, magnetic network of Poland, secular changes of the Earth’s magnetic field, reduction of magnetic declination Received: 29 January 2019 / Accepted: 27 February 2019 Elzbieta Welker Geoinformation Issues Anna Jedrzejewska Vol. 10, No 1 (10), 17–26/2018 1. Introduction elements of the Earth’s magnetic field obtained from the IGRF (International Geomagnetic Reference Changes of the Earth’s magnetic field (a temporal- Field) model and from the results of the absolute spatial physical phenomenon) depend on: magnetic measurements at the repeat stations of na- – the position and module of the central dipole tional magnetic control shows large discrepancies, associated with the chemical composition of especially in the anomalous regions of, for example, the internal core and the speed of its rotation Poland, Lithuania and Belarus. This confirms the relative to the rotation axis of the Earth, justness of the continuation of systematic absolute – the position and module of other internal di- measurements at the repeat stations in Europe poles located around the Earth’s core, (Sas-Uhrynowski and Welker, 2009). Updating mag- – external factors such as the phenomena occurring netic data always requires information about the in the ionosphere and magnetosphere under the normal magnetic field and the course of changes in influence of the Sun, the Moon and other planets, the Earth’s magnetic field in the studied area (Welker and they cannot be modelled. They are treated as and Żółtowski, 1993b) between the measurement random changes in terms of both their speed and epoch and the study epoch. size. This is confirmed by the results of the study of The analysis of the course of the actual changes changes in the position and value of the central dipole of magnetic declination in Poland in the years module and other internal dipoles over a period of one 1954–2017, based on the data from magnetic ob- hundred years (1900–2000). The changes in the servatories and from repeat stations, will make it Earth’s magnetic field generated by them are unpre- possible to unify – reduce to one, current 2017.5 epoch dictable in a longer period of time (e.g. 10–20 years) – all the values of magnetic declination obtained as (Demina et al., 2006). a result of measurement in different epochs, at the In 2003, as part of the MagNetE (Magnetic Net- points of the magnetic network of Poland (Welker, work For Europe) project, joint action concerning 2013), which is part of the magnetic database of magnetic measurements at repeat stations in 20 Euro- the Institute of Geodesy and Cartography. pean countries was undertaken. The results of the observations are used as a basis for the studies of the 2. Historical outline spatial and temporal changes in the Earth’s mag- netic field in Europe and for the development of In the 1950s, the first magnetic network was estab- current magnetic maps for this area (Korte, 2011). lished in Poland at over 3000 points located all over A comparison of the course of the contour lines of the country, with a density appropriate for the occur- A B Fig. 1. Points of the magnetic network from the years 1954–1959 a); points of absolute magnetic measurements from 1965, and from the marine magnetic measurements from the years 1971–1990 b) 18 A study of changes in the Earth's magnetic field in Poland in the years 1954-2017; reducing the archival Geoinformation Issues values of magnetic declination at the points Vol. 10, No 1 (10), 17–26/2018 of the magnetic network to a uniform epoch rence of magnetic anomalies (Fig. 1a). Magnetic declination and the position of the Sun, necessary to determine geographical azimuths of target points were measured at the network points. The points were stabilized only with wooden stakes. In 1965, a full measurement of all three components of the geomagnetic field was repeated at over 500 selected points of the network (Fig. 1b). At the same time, a national magnetic control was created with 20 repeat stations evenly distributed across the country (Fig. 2). These are points which are permanently stabilized in the field, in the places of magnetic “calmness” (without any local anomalous disturbances), at which absolute magnetic measurements of the three inde- pendent components of the Earth’s magnetic field should be systematically repeated. The values of Fig. 2. Points of the national magnetic control (repeat stations) – archival points are marked in red the magnetic declination calculated for the points of the magnetic network and the knowledge of the secular changes in the magnetic field determined epoch t0 = 1974 was adopted, which translated into from the measurements made at the repeat stations the final system of equationsf (x, y, t′): constitute the starting material for the analytical and 2 2 graphic works related to the study of magnetic data. f (x, y, t′) = a1x + a2 y + a3xy + a4x + a5y + a6 + In 1990, absolute magnetic measurements on the + (a x2 + a y2 + a xy + a x + a y + a ) · t′ + (2) Baltic Sea were completed, and the results from 7 8 9 10 11 12 2 2 2 their study, attached to the geophysical database of + (a13x + a14y + a15xy + a16x + a17y + a18) · t′ IGiK (Fig. 1b), broadened significantly the area covered by magnetic measurements (Welker, 2013). where x = (φ – φ0)/dx, y = (λ – λ0)/dy, t’= (t – t0)/dt, dx, At the turn of the 1980s and 1990s, a team of dy are the absolute values of coordinate differences IGiK developed a method of reducing magnetic between the border points of the area and the centre declination based on 40-year long observations at of the local system in degrees, and dt is the difference repeat stations (Welker and Żółtowski, 1993a). of years between the initial epoch and the epoch This method assumed that changes in the magnetic adopted as zero. field at repeat stations can be described by means Additionally, the equations were weighed de- of the polynomial of the 3rd degree, which is a func- pending on the distance of the point from the centre tion of the position of the point (φ, λ) and the of the local system. measurement epoch. The studies were conducted After appropriate calculations, a normal mag- according to the formula: netic field for the area of Poland was obtained (Welker and Żółtowski, 1993b), which was used for 2 2 f (φ, λ, t) = a1φ + a2 λ + a3φλ + a4φ + a5λ + a6 + the reduction of magnetic data and for calculating 2 2 (1) magnetic anomalies. + (a7φ + a8 λ + a9φλ + a10φ + a11λ + a12) · t + After reducing the magnetic declination at the + 2 2 ) · t2 (a13φ + a14λ + a15φλ + a16φ + a17λ + a18 points of the magnetic network of Poland to the 2000.5 epoch based on the actual changes of mag- where φ, λ are the coordinates of the repeat station netic declination in observatories and at repeat sta- in the WGS-84 system, t is the measurement epoch, tions, and using the polynomial function, it was and ai are coefficients to be determined.