Pavel Kostecký

A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra Observatory

A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra Observatory The Schumann resonances (SR) are electromagnetic eigenmodes of the resonator bounded by the Earths 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.

On the relation between the red sprites and the transients in the ELF band

On the relation between the red sprites and the transients in the ELF band Under favourable conditions sprites appear above large storms. Long continuing current in tens of ms in the parent +CG stroke radiates electromagnetic energy also in the Schumann resonance (SR) band. Optical and Extremely Low Frequency (ELF) observations at the Astronomical and Geophysical Observatory (AGO) near Modra are analyzed to find these two effects associated with +CG discharges. Since April 2007 dozens of sprites have been captured by automated all-sky TV system at AGO. A majority (77%) of the optical transient events are accompanied with the transients in the SR band. No ELF counterparts are found for 23% of the captured sprites. Our observations are compared with similar observations in Sopron and Nagycenk (NCK). Example of the optical frames, the associated ELF time plot and the Fourier as well as dynamic spectrum are presented for one of the events simultaneously observed at AGO and NCK.

The Estimation of the Cavity Effect by Higher Degree Finite Element Approximation

The measurements of tide-induced tilts and deformations are usually performed in underground cavities (mostly man-made). Therefore, the data obtained must be debugged of the influence of the cavity itself (cavity effect) and of the effect of elastic parameter inhomogeneities in the vicinity of the point of measurement (geological effect), if they are to he presented. Since these effects are reflected as a modulation of tidal waves (in both amplitude and phase), the only way to perform this can be modelling of the strain and stress field around the cavity as precisely as possible. The finite element method (F.E.M.) seems to be a very useful tool for this purpose. To justify some conclusions in this paper, a number of numerical runs of the F.E.M. models of the tide-induced strains and stresses (in two dimensions) were performed. A higher degree of approximation (up to the value of 7) was used in these calculations. To estimate the cavity effect, the correction factors (relative changes of particular strain components due to the presence of the cavity) were determined and plotted in the cavitys close neighbourhood. The distribution of the deformation energy (and also the work of external forces) over the whole domain of interest is given, and the conclusions for subsequent error estimation are drawn. An attempt was made to compare the real tilt data to the theoretical deduced from the numerical model.

Simultaneous Measurements of Low-Frequency Natural Electric Field at Two Neighbouring Stations

Simultaneous Measurements of Low-Frequency Natural Electric Field at Two Neighbouring Stations The electromagnetic eigenmodes of Earth-ionosphere global resonator known as Schumann Resonances (SR) are excited by global lightning activity. The simultaneous measurements of electric field in the ELF (~5 - 100 Hz) frequency band at two observatories clearly reveal the global character of the resonator response to the excitation both in the permanent Schumann background as well as in isolated events (transients). It is shown that measurement of ELF electric field can be done with relatively simple equipment.

Modelling of the rock structure stress field near the cavities and estimation of the cavity effect influence on the tidal measurements

The article is dealing with the problem of cavity effect influence on the tidal measurements in geophysics. The measurements of tide-induced deformations and tilts are usually performed in underground cavities. The data obtained must be debugged of the influence of the cavity itself (cavity effect) and of the effect of elastic parameter inhomegeneities in the vicinity of the measurement point (geological effect). Since these effects are reflected as a modulation of tidal waves (in both amplitude and phase), the most natural way to perform this can be modelling of the strain and stress field around the cavity as precisely as possible. Separation of the above mentioned effects from measurements by the time series analysis is principally not possible.