Analysis of Croatian geomagnetic repeat station surveys in 2004 and 2007

Abstract

Surveys of the geomagnetic declination, inclination and total field intensitywere carried out at eight stations of the Croatian Geomagnetic Repeat Stations Network in 2004 and 2007. The total field time series at repeat stations and observatories were compared by use of the multi-linear regression and linear correlation coefficients. The surveys' data were reduced to 2004.5 and 2007.5 epochs by a simple method proposed here, assuming the secular variations at the repeat station and observatory are equal or different, respectively. These reductions, obtained in reference to L'Aquila, Fürstenfeldbruck and Tihany observatories, and corresponding weighted as well as nonweighted averages, were compared. The preferred geomagnetic field elements as well as the corresponding annual changes between 2004.5 and 2007.5 epochs, were compared to the latest International Geomagnetic Reference Field. Additionally, the Croatian Geomagnetic Normal Reference Fields for 2004.5 and 2007.5 epochs were presented.

Croatian geomagnetic field maps for 2008.5 epoch

“…The actual procedure of the mapping stations' setup and survey was similar to the one in case of repeat stations [Brkić et al 2006, Vujić et al 2011; however some criteria were relaxed, e.g. low gradient criteria was set to 5 nT/m; the laborious outer grid gradiometry method was replaced by a simpler and quicker modified cross gradiometry method (a method based on cross method [Newitt et al 1996]); gradiometer measurements are performed in 10 m × 10 m NS and WE lines with 1 m step.…”

“…Each individual observation was reduced to the epoch 2008.5 and in reference to nearby observatory using INTERMAGNET data, and averaged to obtain the particular representative geomagnetic element value at the station. The similar procedure was applied to the reduction of 2004 and 2007 repeat station data [Vujić et al 2011]; since the expected difference between the simple reduction method, and the method in which the secular variation differs at the station and observatory, was not large (the maximum differences between the averaged solutions obtained by these two methods being 11" for D, 7" for I and 0.5 nT for F for epoch 2007.5), the simple method was chosen to reduce 2008 measurements.…”

“…As in Vujić et al [2011], the appropriateness of data for the reduction was checked by cross-correlation of total in-…”

“…As an estimation of the main field, the Croatian Geomagnetic Normal Reference Field (CGNRF) for the epoch 2008.5 was obtained from the Table 2, and the L'Aquila (AQU), Fürstenfeldbruck (FUR), Tihany (THY), Grocka (GCK) and Panagjurishte (PAG) observatories 2008.5 annual means similarly to Vujić et al [2011]. The geomagnetic element E normal was represented by the second order polynomial in latitude and longitude:…”

Recent Efforts Toward the Establishment of the Lonjsko Polje Geomagnetic Observatory

“…Since then, Croatia has lacked observatory measurements on its territory. In the last decade, the Croatian Geomagnetic Repeat Station Network (CGRSN, 10 stations) together with the denser Croatian Geomagnetic Network for Field Mapping (CGNFM, 88 stations) has been established and surveyed (Brkiü et al 2006, Brkiü and Šugar 2008, Vujiü et al 2011, Brkiü et al 2012. The purpose of these measurements is to renew the geomagnetic field information on Croatian territory and reveal the small-scale field variations produced by nearsurface magnetic rocks.…”