[1] The paper presents a summary of more than 4 years of continuous Schumann resonance (SR) monitoring of the vertical electric component at Modra Observatory. Principal parameters (peak frequency, amplitude, and quality factor) are determined for four resonances from 7 to 30 Hz, i.e., for modes one through four. Attention is also given to the less frequently compiled mode four. The resonance parameters are computed from 48 daily measurements and are represented as the mean monthly values for each time of the detection. Fitting of spectral peaks by Lorentz function is the main method used in data postprocessing. Diurnal, seasonal variations and the indication of interannual variations of these parameters (especially peak frequency) are discussed. The results are compared with other observatory measurements. Our observations confirm that variations in peak frequency of the lower-SR modes can be attributed mainly to the source-observer distance effect.

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Linear magnetoconvection in a model of a non-uniformly stratified horizontal rotating fluid layer with a toroidal magnetic field is investigated for no-slip and finitely electrically conductive boundaries and with very thin stably stratified upper sublayer. The basic parabolic temperature profile is determined by the temperature difference between the boundaries and by the homogeneous heat source distribution in the layer. This results in a density pattern, in which a stably stratified upper sublayer is present. The developed diffusive perturbations (modes) are strongly affected by the complicated coupling of viscous, thermal and magnetic diffusive processes. The calculations were performed for various values of Roberts number (q ≪ 1 and q = O(1)). The mean electromotive force produced by the developed hydromagnetic instabilities is investigated to find the modes, which can be appropriate for creating the α-effect. It was found that the azimuthal part of the EMF is dominant for westward modes when the Elsasser number Λ ∼ < O(1).K e y w o r d s : non-uniform stratification of the Earth's core, linear magnetoconvection, diffusive hydromagnetic instabilities, mean electromotive force Stud. Geophys. Geod., 50 (2006), 645-661 645 c 2006 StudiaGeo s.r.o., Prague J.Šimkanin et al.

A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra ObservatoryThe Schumann resonances (SR) are electromagnetic eigenmodes of the resonator bounded by the Earth's surface and the lower ionosphere. The SR frequency variability has been studied for more than 4 decades. Using data from the period 1988 to 2002,Sátori et al. (2005)showed that the SR fundamental mode frequency decreased on the 11-year time scale by 0.07 - 0.2 Hz, depending on which component of the field was used for estimation and likely also on the location of the observer. A decrease by 0.30 Hz from the latest solar cycle maximum to the minimum of 2009 is found in data from Modra Observatory. This extraordinary fall of the fundamental mode frequency can be attributed to the unprecedented drop in the ionizing radiation in X-ray frequency band. Although the patterns of the daily and seasonal variations remain the same in the solar cycle minimum as in the solar cycle maximum, they are significantly shifted to lower frequencies during the minimum. Analysis of the daily frequency range suggests that the main thunderstorm regions during the north hemisphere summer are smaller in the solar cycle minimum than in the maximum.

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