Bidzina M. Shergelashvili

Rossby waves in "shallow water" magnetohydrodynamics

Aims. The influence of a toroidal magnetic field on the dynamics of Rossby waves in a thin layer of ideal conductive fluid on a rotating sphere is studied in the “shallow water” magnetohydrodynamic approximation for the first time. Methods. Dispersion relations for magnetic Rossby waves are derived analytically in Cartesian and spherical coordinates. Results. It is shown that the magnetic field causes the splitting of low order (long wavelength) Rossby waves into two different modes, here denoted fast and slow magnetic Rossby waves. The high frequency mode (the fast magnetic Rossby mode) corresponds to an ordinary hydrodynamic Rossby wave slightly modified by the magnetic field, while the low frequency mode (the slow magnetic Rossby mode) has new and interesting properties since its frequency is significantly smaller than that of the same harmonics of pure Rossby and Alfven waves.

A Selection of Nonequilibrium Issues

We give a pedagogical introduction to a selection of recently discussed topics in nonequilibrium statistical mechanics, concentrating mostly on formal structures and on general principles. Part I contains an overview of the formalism of lattice gases that we use to explain various symmetries and inequalities generally valid for nonequilibrium systems, including the fluctuation symmetry, Jarzynski equality, and the direction of currents. That mostly concerns the time-antisymmetric part of dynamical fluctuation theory.We also briefly comment on recent attempts to combine that with the time-symmetric sector in a Langrangian or extended Onsager-Machlup approach. In Part II we concentrate on the macroscopic state and how entropy provides a bridge between microscopic dynamics and macroscopic irreversibility; included is a construction of quantum macroscopic states and a result on the equivalence of ensembles.

Acoustic oscillations in a field-free cavity under solar small-scale bipolar magnetic canopy

Observations show the increase of high-frequency wave power near magnetic network cores and active regions in the solar lower atmosphere. This phenomenon can be explained by the interaction of acoustic waves with a mag- netic field. We consider small-scale, bipolar, magnetic field canopy structure near the network cores and active regions overlying field-free cylindrical cavities of the photosphere. Solving the plasma equations we get the analytical disper- sion relation of acoustic oscillations in the field-free cavity area. We found that the m=1 mode, where m is azimuthal wave number, cannot be trapped under the canopy due to en- ergy leakage upwards. However, higher (m 2) harmonics can be easily trapped leading to the observed acoustic power halos under the canopy.

Nonmodal Cascade in the Compressible Solar Atmosphere: Self-Heating, an Alternative Way to Enhance Wave Heating

The nonmodal self-heating mechanism recently proposed by Rogava is applied to the medium of solar coronal holes with an inhomogeneous plasma flow along the magnetic field lines. The viscosity force is assumed to be anisotropic. The efficiency of the nonmodal cascade process is examined for different sets of environmental parameters and for different wave parameters. It is concluded that the proposed mechanism can serve as an alternative mechanism for explaining the significant heat production in the lower corona, even when only laminar values of the viscosity coefficients are taken into account.

Acoustic oscillations in the field-free, gravitationally stratified cavities under solar bipolar magnetic canopies

Aims. The main goal here is to study the dynamics of the gravitationally stratified, field-free cavities in the solar atmosphere, located under small-scale, cylindrical magnetic canopies, in response to explosive events in the lower-lying regions (due to granulation, smallscale magnetic reconnection, etc.). Methods. We derive the two-dimensional Klein-Gordon equation for isothermal density perturbations in cylindrical coordinates. The equation is first solved by a standard normal mode analysis to obtain the free oscillation spectrum of the cavity. Then, the equation is solved in the case of impulsive forcing associated to a pressure pulse specified in the lower lying regions. Results. The normal mode analysis shows that the entire cylindrical cavity of granular dimensions tends to oscillate with frequencies of 5–8 mHz and also with the atmospheric cut-off frequency. Furthermore, the passage of a pressure pulse, excited in the convection zone, sets up a wake in the cavity oscillating with the same cut-off frequency. The wake oscillations can resonate with the free oscillation modes, which leads to an enhanced observed oscillation power. Conclusions. The resonant oscillations of these cavities explain the observed power halos near magnetic network cores and active regions.

Rieger-type periodicity during solar cycles 14-24: estimation of dynamo magnetic field strength in the solar interior

Solar activity undergoes a variation over time scales of several months known as Rieger-type periodicity, which usually occurs near maxima of sunspot cycles. An early analysis showed that the periodicity appears only in some cycles, and is absent in other cycles. But the appearance/absence during different cycles has not been explained. We performed a wavelet analysis of sunspot data from the Greenwich Royal Observatory and the Royal Observatory of Belgium during cycles 14-24. We found that the Rieger-type periods occur in all cycles, but they are cycle-dependent: shorter periods occur during stronger cycles. Our analysis revealed a periodicity of 185-195 days during the weak cycles 14-15 and 24, and a periodicity of 155-165 days during the stronger cycles 16-23. We derived the dispersion relation of the spherical harmonics of the magnetic Rossby waves in the presence of differential rotation and a toroidal magnetic field in the dynamo layer near the base of the convection zone. This showed that the harmonic of fast Rossby waves with m=1 and n=4, where m (n) indicate the toroidal (poloidal) wavenumbers, respectively, perfectly fit with the observed periodicity. The variation of the toroidal field strength from weaker to stronger cycles may lead to the different periods found in those cycles, which explains the observed enigmatic feature of the Rieger-type periodicity. Finally, we used the observed periodicity to estimate the dynamo field strength during cycles 14-24. Our estimations suggest a field strength of 40 kG for the stronger cycles, and 20 kG for the weaker cycles.

Formation and evolution of coronal rain observed by SDO/AIA on February 22, 2012

The formation and dynamics of coronal rain are currently not fully understood. Coronal rain is the fall of cool and dense blobs formed by thermal instability in the solar corona towards the solar surface with acceleration smaller than gravitational free fall. We aim to study the observational evidence of the formation of coronal rain and to trace the detailed dynamics of individual blobs. We used time series of the 171 \AA\, and 304 \AA\, spectral lines obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) above active region AR 11420 on February 22, 2012. Observations show that a coronal loop disappeared in the 171 \AA\ channel and appeared in the 304 \AA\ line

ON THE LOW-FREQUENCY BOUNDARY OF SUN-GENERATED MAGNETOHYDRODYNAMIC TURBULENCE IN THE SLOW SOLAR WIND

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Nonequilibrium relation between potential and stationary distribution for driven diffusion

more than one hour later, which indicates a rapid cooling of the coronal loop from 1 MK to 0.05 MK. An energy estimation shows that the radiation is higher than the heat input, which indicates so-called catastrophic cooling. The cooling was accompanied by the formation of coronal rain in the form of falling cold plasma. We studied two different sequences of falling blobs. The first sequence includes three different blobs. The mean velocities of the blobs were estimated to be 50 km s

DYNAMICS OF A SOLAR PROMINENCE TORNADO OBSERVED BY SDO/AIA ON 2012 NOVEMBER 7–8

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