On the Seasonal Quiet Daily Variations at the Lonjsko Polje Geomagnetic Observatory (Croatia)

Abstract

The behaviour of the quiet daily variations during different levels of solar activity over midlatitudes in Europe was the motivation for this study. The main aim was to determine the quiet daily variations from the Lonjsko Polje observatory (Croatia), for the first time. The hourly mean values of the three orthogonal components from 2013 to 2023 were used to examine and build an empirical model of the quiet daily variations for Lloyd’s seasons. An empirical model with 1071 coefficients per component was developed based on the assumptions that it depends on local time, lunar age, season, and solar activity, represented by solar radio flux at 10.7 cm. The obtained model suggests that amplitude levels rise, and phase variations are affected by increasing solar activity. The derived seasonal variations were compared to the variations from the Fürstenfeldbruck, Panagjurishte and Tihany observatories during 2013−2021. Due to the absence of data from Lonjsko Polje during periods of higher solar activity, seasonal variations were examined using records from Fürstenfeldbruck and Panagjurishte over the period 1956−2021 and Tihany over 1997−2021. The modelled patterns of seasonal variations with different levels of solar activity showed good agreement with the observations. The seasonal variations from Fürstenfeldbruck and Panagjurishte averaged over each of six consecutive nonoverlapping 11-year intervals during 1956−2021 are highly correlated, while some scatter up to 7 nT is observed for each geomagnetic component, depending on local time. The seasonal variations at Lonjsko Polje for the period of 1956 to 2012 were reconstructed using a multilinear regression analysis of data from Fürstenfeldbruck and Panagjurishte, with the maximum amplitudes of 30, 90, and 40 nT for the north, east, and vertical components, respectively. The results from this study could lead to a possible investigation of the equivalent ionospheric and induced currents responsible for derived variations.