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Tevzadze A.G.,Tbilisi State University | Chagelishvili G.D.,Tbilisi State University | Chagelishvili G.D.,Nodia Institute of Geophysics | Bodo G.,National institute for astrophysics | Rossi P.,National institute for astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

We investigate mode coupling in a two-dimensional compressible disc with radial stratification and differential rotation. We employ the global radial scaling of linear perturbations and study the linear modes in the local shearing-sheet approximation. We employ a three-mode formalism and study the vorticity (W), entropy (S) and compressional (P) modes and their coupling properties. The system exhibits asymmetric three-mode coupling: this includes mutual coupling of S and P modes, S and W modes, and asymmetric coupling between the W and P modes. P-mode perturbations are able to generate potential vorticity through indirect three-mode coupling. This process indicates that compressional perturbations can lead to the development of vortical structures and influence the dynamics of radially stratified hydrodynamic accretion and protoplanetary discs. © 2009 RAS.

Eftaxias K.,National and Kapodistrian University of Athens | Potirakis S.M.,Technological Educational Institute of Piraeus | Chelidze T.,Nodia Institute of Geophysics
Natural Hazards and Earth System Sciences | Year: 2013

It has been suggested that fracture-induced MHz-kHz electromagnetic emissions (EME), which emerge from a few days up to a few hours before the main seismic shock occurrence permit a real-time monitoring of the damage process during the last stages of earthquake preparation, as it happens at the laboratory scale. Despite fairly abundant evidence, electromagnetic (EM) precursors have not been adequately accepted as credible physical phenomena. These negative views are enhanced by the fact that certain "puzzling features" are repetitively observed in candidate fracture-induced pre-seismic EME. More precisely, EM silence in all frequency bands appears before the main seismic shock occurrence, as well as during the aftershock period. Actually, the view that "acceptance of "precursive" EM signals without convincing co-seismic signals should not be expected" seems to be reasonable. In this work we focus on this point. We examine whether the aforementioned features of EM silence are really puzzling ones or, instead, reflect well-documented characteristic features of the fracture process, in terms of universal structural patterns of the fracture process, recent laboratory experiments, numerical and theoretical studies of fracture dynamics, critical phenomena, percolation theory, and micromechanics of granular materials. Our analysis shows that these features should not be considered puzzling. © 2013 Author(s).

Mamatsashvili G.R.,Tbilisi State University | Chagelishvili G.D.,Ilia State University | Chagelishvili G.D.,Nodia Institute of Geophysics | Bodo G.,National institute for astrophysics | Rossi P.,National institute for astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the linear dynamics of non-axisymmetric perturbations in incompressible, vertically stratified Keplerian discs threaded by a weak non-zero net vertical magnetic field in the local shearing box approximation. Perturbations are decomposed into shearing waves or spatial harmonics whose temporal evolution is then followed via numerical integration of the linearized ideal magnetohydrodynamic equations of the shearing box. There are two basic modes in the system - inertia-gravity waves and magnetic mode, which displays the magnetorotational instability (MRI). Distinct from previous related studies, we introduce 'eigenvariables' characterizing each (counter-propagating) component of the inertia-gravity and magnetic modes, which are governed by a set of four first-order coupled ordinary differential equations. This allows us to identify a new process of linear coupling of the two above nonaxisymmetric modes due to the disc's differential rotation. We also carry out a comparative analysis of the dynamics of non-axisymmetric and axisymmetric magnetic mode perturbations. It is demonstrated that the growth of 'optimal' and close-to-optimal non-axisymmetric harmonics of this mode, having transient nature, can prevail over the exponential growth of axisymmetric ones (i.e. over the axisymmetric MRI) during dynamical time. A possible implication of this result for axisymmetric channel solutions emerging in numerical simulations is discussed. In particular, the formation of the (axisymmetric) channel may be affected/impeded by non-axisymmetric modes already at the early linear stage leading to its untimely disruption - the outcome strongly depends on the amplitude and spectrum of initial perturbation. Thus, this competition may result in an uncertainty in the magnetic mode's non-linear dynamics. Even so, we consider that incompressible perturbations, in the final part, speculate on the dynamics in the compressible case. It is shown that a maximum growth of non-axisymmetric magnetic mode occurs at vertical wavelengths close to the disc scaleheight, for which compressibility effects are important. This indirectly suggests that compressibility plays a role in the dynamics of the non-axisymmetric MRI and, ultimately, in the resulting turbulent state. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kordzadze A.A.,Nodia Institute of Geophysics | Demetrashvili D.I.,Nodia Institute of Geophysics
Ocean Science | Year: 2011

One of the parts of the Black Sea Nowcasting/Forecasting System is the regional forecasting system for the easternmost part of the Black Sea (including the Georgian water area), which has been developed within the context of the EU International projects ARENA and ECOOP. A core of the regional system is a high-resolution baroclinic regional model of the Black Sea dynamics developed at M. Nodia Institute of Geophysics (RM-IG). This model is nested in the basin-scale model of Marine Hydrophysical Institute (MHI, Sevastopol/Ukraine). The regional area is limited to the Caucasian and Turkish coastal lines and the western liquid boundary coinciding with the meridian 39.36 E. Since June 2010 we have regularly been computing 3 days' forecasts of current, temperature and salinity for the easternmost part of the Black Sea with 1 km spacing. In this study the results of two forecasts are presented. The first forecast corresponds to summer season and covers the prognostic interval from 00:00 h, 6 August to 00:00 h, 9 August 2010. The second one corresponds to autumn season and covers the prognostic interval from 00:00 h, 26 October to 00:00 h, 29 October 2010. Data needed for the forecasts-the initial and prognostic hydrophysical fields on the open boundary, also 2-D prognostic meteorological fields at the sea surface-wind stress, heat fluxes, evaporation and precipitation rates for our regional area are being placed on the MHI server every day and we are available to use these data operatively. Prognostic hydrophysical fields are results of forecast by the basin-scale model of MHI and 2-D meteorological boundary fields represent the results of forecast by regional atmospheric model ALADIN. All these fields are given on the grid of basin-scale model with 5 km spacing and with one-hour time step frequency for the integration period. The analysis of predicted fields shows that to use the model with high resolution is very important factor for identification of nearshore eddies of small sizes. It should be noted the very different character of regional circulation in summer and autumn seasons in the easternmost part of the Black Sea. © 2013 Author(s).

Telesca L.,CNR Institute of Methodologies for Environmental analysis | Matcharashvili T.,Nodia Institute of Geophysics | Chelidze T.,Nodia Institute of Geophysics
Natural Hazards and Earth System Science | Year: 2012

The time-clustering behaviour of the seismicity of the Caucasus spanning from 1960 to 2010 was investigated. The analysis was performed on the whole and aftershock-depleted catalogues by means of the method of Allan Factor, which permits the identification and quantification of time-clustering in point processes. The whole sequence is featured by two scaling regimes with the scaling exponent at intermediate timescales lower than that at high timescales, and a crossover that could be probably linked with aftershock time activiation. The aftershock-depleted sequence is characterized by higher time-clustering degree and the presence of a periodicity probably correlated with the cyclic earth surface load variations on regional and local scales, e.g. with snow melting in Caucasian mountains and large Enguri dam operations. The obtained results were corroborated by the application of two surrogate methods: the random shuffling and the generation of Poissonian sequences. © 2012 Author(s).

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