Mario Brkić

Data reduction of Croatian geomagnetic repeat stations surveys by using the spherical elementary current systems method

The geomagnetic declination, inclination and total field data surveyed at the Croatian Geomagnetic Repeat Stations Network were reduced to 2008.5, 2009.5 and 2010.5 epochs by a well-known observatory-based and a proposed method, in instances when an influence of the differences of secular variations at a repeat station and at an observatory were not taken into account. The motivation came from distances and distribution of the nearby geomagnetic observatories. A technique for expanding vector systems on a sphere into basis functions, known as the Spherical Elementary Current System (SECS) method, was applied to obtain separation of the geomagnetic field variations into external and internal parts, over Croatia and surrounding countries. The aim of this work is to propose the SECS method for determination of the annual mean values of north, east and vertical components of Earth’s magnetic field at the Croatian repeat stations. The results of the SECS method were compared to an observatory-based method which assumes that the variations of components about their annual mean values at a repeat station are equal to those at a particular nearby reference observatory. For reducing a repeat station data by this simple method the Fürstenfeldbruck, Grocka and Tihany observatories were used, while for the SECS method additional three observatories were employed. The reduction methods were found comparable, while in the majority of instances the SECS method demonstrated smaller standard deviations of the components’ annual means. Besides, the annual mean values of the vertical component had the highest accuracies for both methods. The results indicate that the SECS method can be an acceptable method for data reduction to annual mean values.

Regional Geomagnetic Field Model for Croatia at 2009.5

Geomagnetic data of north, east, and vertical components at Croatian repeat stations and ground survey sites, as well as European geomagnetic observatories and repeat stations, were used to obtain a regional geomagnetic model over Croatia at 2009.5 epoch. Different models were derived, depending on input data, and three modelling techniques were used: Taylor Polynomial, Adjusted Spherical Harmonic Analysis, and Spherical Harmonic Analysis. It was derived that the most accurate model over Croatia was the one when only Croatian data were used, and by using the Adjusted Spherical Harmonic Analysis. Based on Croatian repeat stations’ data in the interval 2007.5–2010.5, and a global Enhanced Magnetic Model, it was possible to estimate the crustal field at those sites. It was also done by taking into account the empirical adjustment for long-term external field variations. The higher crustal field values were found at those stations which are on or close to the Adriatic anomaly.

Estimation of effect of unequal secular variation on geomagnetic data reduction

The geomagnetic field components are periodically measured at repeat stations. The main objective of the repeat stations is to provide data for tracing the secular variation of the geomagnetic field components. Secular variation at the repeat station is generally different from that at geomagnetic observatory used in the data reduction. The effect of the secular variation differences on geomagnetic data reduction was estimated for the regions of Europe, North America (below 60°N) and Australia, respectively, during the period of 2000-2010. These estimations were obtained by using the monthly mean values of north, east and vertical components of geomagnetic field, recorded at geomagnetic observatories. The effects were calculated by using observatories pairs, with distances from 350 km (in Europe) to 3100 km (in North America and Australia). The maximal effects were found to be the smallest in east component in Europe and North America, and vertical component in Australia; the effects increase with time from a central reduction epoch and they are not constant during mentioned eleven years; they were less than 1 nT only in Europe, for distances between the observatories up to 1000 km in all three components and for periods spanning ±1 month from a central epoch. It was found that their year to year variability is mostly due to the non-eliminated external field residuals in the observatories monthly means; their effect is up to 3 nT for ±3 months from a central epoch. Further, the real effects were compared to those modeled by IGRF-12 model. The maximal differences between the real and the modeled values are below 4.5 nT in all three components, for ±3 months from a central epoch.

Spherical Elementary Current Systems Method Applied to Geomagnetic Field Modeling for the Adriatic

The aim of this work was to derive an accurate regional model of geomagnetic components on the Adriatic. Data of north, east and vertical geomagnetic components at repeat stations and ground survey sites enclosing the Adriatic Sea were used to obtain a geomagnetic model at 2010.5 epoch. The core field was estimated by use of the global Enhanced Magnetic Model, while the crustal field by a mathematical technique for expanding vector systems on a sphere into basis functions, known as spherical elementary current systems method. The results of this method were presented and compared to the crustal field estimations by the Enhanced Magnetic Model. The maps of isolines of the regional model are also presented.