arXiv subjects

D. Tsiklauri

Publications and source records attributed to D. Tsiklauri.

49 records · Page 3Linked to original sources

arXiv2004

Acoustic Oscillations in Solar and Stellar Flaring Loops

Evolution of a coronal loop in response to an impulsive energy release is numerically modelled. It is shown that the loop density evolution curves exhibit quasi-periodic perturbations with the periods given approximately by the ratio of the loop length to the average sound speed, associated with the second standing harmonics of an acoustic wave. The density perturbations have a maximum near the loop apex. The corresponding field-aligned flows have a node near the apex. We suggest that the quasi-periodic pulsations with periods in the range 10-300 s, frequently observed in flaring coronal loops in the radio, visible light and X-ray bands, may be produced by the second standing harmonic of the acoustic mode.

astro-ph↗

arXiv2003

Phase mixing of a three dimensional magnetohydrodynamic pulse

Phase mixing of a three dimensional magnetohydrodynamic (MHD) pulse is studied in the compressive, three-dimensional (without an ignorable coordinate) regime. It is shown that the efficiency of decay of an Alfvenic part of a compressible MHD pulse is related linearly to the degree of non-planeness of the pulse in the homogeneous transverse direction. In the developed stage of phase mixing (for large times), coupling to its compressive part does not alter the power-law decay of an Alfvenic part of a compressible MHD pulse. The same applies to the dependence upon the resistivity of the Alfvenic part of the pulse. All this implies that the dynamics of Alfven waves can still be qualitatively understood in terms of the previous 2.5D models. Thus, the phase mixing remains a relevant paradigm for the coronal heating applications in the realistic 3D geometry and compressive plasma.

astro-ph↗

arXiv2002

A Strongly Nonlinear Alfvenic Pulse in a Transversely Inhomogeneous Medium

We investigate the interaction of a plane, linearly polarized, Alfvenic pulse with a one-dimensional, perpendicular to the magnetic field, plasma density inhomogeneity in the strongly nonlinear regime. Our numerical study of the full MHD equations shows that: (i) Plasma density inhomogeneity substantially enhances (by about a factor of 2) the generation of longitudinal compressive waves. (ii) Attained maximal values of the generated transverse compressive perturbations are insensitive to the strength of the plasma density inhomogeneity, plasma beta and the initial amplitude of the Alfven wave. Typically, they reach about 40% of the initial Alfven wave amplitude. (iii) Attained maximal values of the generated relative density perturbations are within 20-40% for 0.5 <= beta <= 2.0. They depend upon plasma beta strongly; and scale almost linearly with the initial Alfven wave amplitude.

astro-ph↗

arXiv2002

Line-of-sight Effects on Observability of Kink and Sausage Modes in Coronal Structures with Imaging Telescopes

Kink modes of solar coronal structures, perturbing the loop in the direction along the line-of-sight (LOS), can be observed as emission intensity disturbances propagating along the loop provided the angle between the LOS and the structure is not ninety degrees. The effect is based upon the change of the column depth of the loop (along the LOS) by the wave. The observed amplitude of the emission intensity variations can be larger than the actual amplitude of the wave by a factor of two and there is an optimal angle maximizing the observed amplitude. For other angles this effect can also attenuate the observed wave amplitude. The observed amplitude depends upon the ratio of the wave length of kink perturbations to the width of the structure and on the angle between the LOS and the axis of the structure. Sausage modes are always affected negatively from the observational point of view, as the observed amplitude is always less than the actual one. This effect should be taken into account in the interpretation of wave phenomena observed in the corona with space-borne and ground-based imaging telescopes.

astro-ph↗

arXiv2002

A Three Dimensional Magnetohydrodynamic Pulse in a Transversely Inhomogeneous Medium

Interaction of impulsively generated MHD waves with a one-dimensional plasma inhomogeneity, transverse to the magnetic field, is considered in the three-dimensional regime. Because of the transverse inhomogeneity, MHD fluctuations, even if they do not include initially any density perturbation, evolve toward states where the compressible components tend to become predominant. The propagating MHD pulse asymptotically reaches a quasi-steady state with the final levels of density perturbation weakly depending on the degree of non-planeness of the pulse in the homogeneous transverse direction and somewhat stronger depending on plasma beta. Our study demonstrates the necessity of incorporation of compressible and 3D effects in theory of Alfven wave phase mixing. However, as far as the dynamics of weakly non-plane Alfven waves is concerned it can still be qualitatively understood in terms of the previous 2.5D models.

astro-ph↗

arXiv2002

Properties of Elastic Waves in a non-Newtonian (Maxwell) Fluid-Saturated Porous Medium

The present study investigates novelties brought about into the classic Biot's theory of propagation of elastic waves in a fluid-saturated porous solid by inclusion of non-Newtonian effects that are important, for example, for hydrocarbons. Based on our previous results (Tsiklauri and Beresnev: 2001, Phys. Rev. E, 63, 046304), we have investigated the propagation of rotational and dilatational elastic waves, through calculating their phase velocities and attenuation coefficients as a function of frequency. We found that the replacement of an ordinary Newtonian fluid by a Maxwell fluid in the fluid-saturated porous solid results in: (a) an overall increase of the phase velocities of both the rotational and dilatational waves. With the increase of frequency these quantities tend to a fixed, higher, as compared to the Newtonian limiting case, level which is not changing with the decrease of the Deborah number $α$. (b) the overall decrease of the attenuation coefficients of both the rotational and dilatational waves. With the increase of frequency these quantities tend to a progressively lower, as compared to the Newtonian limiting case, levels as $α$ decreases. (c) Appearance of oscillations in all physical quantities in the deeply non-Newtonian regime.

physics.geo-ph↗

arXiv2001

Non-resonant nonlinear coupling of magnetohydrodynamic waves in inhomogeneous media

A new mechanism for the enhanced generation of compressible fluctuations by Alfven waves is presented. A strongly nonlinear regime of Alfven wave phase-mixing is numerically simulated in a one-dimensionally inhomogeneous plasma of finite temperature. It is found that the inhomogeneity of the medium determines the efficiency of nonlinear excitation of magnetoacoustic waves. The level of the compressible fluctuations is found to be higher (up to the factor of two) in inhomogeneous regions. The amplitude of the generated magnetoacoustic wave can reach up to 30% of the source Alfven wave amplitude, and this value is practically independent of the Alfven wave amplitude and the steepness of Alfven speed profile. The highest amplitudes of compressible disturbances are reached in plasmas with beta of about 0.5. The further growth of the amplitude of compressible fluctuations is depressed by saturation.

astro-ph↗

arXiv2001

Nonlinear MHD Waves in the Solar Wind Plasma Structures

We investigate the interaction of magnetohydrodynamic waves with plasma density inhomogeneities. Our numerical study of the full MHD equations shows that: (A) Plasma density inhomogeneities are a source of non-linear generation of transverse compressive waves by a plane Alfven wave, and substantially enhance (by about a factor of 2) the generation of the longitudinal compressive waves. (B) Attained maximal values of the generated transverse compressive waves are insensitive to the strength of the plasma density inhomogeneity and the initial amplitude of the Alfven wave. (C) Efficiency of the generation depends weakly upon the plasma beta parameter. The maximum generated amplitude of transverse compressive wave, up to 32% of the initial Alfven wave amplitude, is reached for about beta=0.5. The results obtained demonstrate that plasma inhomogeneities enhance the efficiency of the non-linear wave coupling.

astro-ph↗

arXiv2001

Wide-Spectrum Slow Magnetoacoustic Waves in Coronal Loops

A model interpreting variations of EUV brightness upward propagating in solar coronal loops as slow magnetoacoustic waves is developed. A loop is considered to have a non-zero plane inclination angle and offset of circular loop centre from the baseline. The model also incorporates effects of dissipation and gravitational stratification. A linear evolutionary equation is derived and applied to investigation of slow wave dynamics. Both the non-zero plane inclination and the offset decrease the growth of the wave amplitude due to stratification. It is shown that wide spectrum slow magnetoacoustic waves, consistent with currently available observations in the low frequency part of the spectrum, can provide the rate of heat deposition sufficient to heat the loop. In this scenario, the heat would be deposited near the loop footpoints which agrees with the current observational data.

astro-ph↗

arXiv2001

A Weakly Nonlinear Alfvenic Pulse in a Transversely Inhomogeneous Medium

The interaction of a weakly nonlinear Alfvénic pulse with an Alfvén speed inhomogeneity in the direction perpendicular to the magnetic field is investigated. Identically to the phase mixing experienced by a harmonic Alfvén wave, sharp transverse gradients are generated in the pulse by the inhomogeneity. In the initial stage of the evolution of an initially plane Alfvénic pulse, the transverse gradients efficiently generate transversely propagating fast magnetoacoustic waves. However, high resolution full MHD numerical simulations of the developed stage of the pulse evolution show that the generation saturates due to destructive wave interference. It is shown that the weakly non-linear description of the generated fast magnetoacoustic wave is well described by the driven wave equation proposed in Nakariakov, Roberts & Murawski (1997), and a simple numerical code (2D MacCromack), which solves it with minimal CPU resources produces identical results to those obtained from the full MHD code ({\it Lare2d}, Arber et al. 2001). A parametric study of the phenomenon is undertaken, showing that, contrary to one's expectations, steeper inhomogenities of the Alfvén speed do not produce higher saturation levels of the fast wave generation. There is a certain optimal gradient of the inhomogeneity that ensures the maximal efficiency of the fast wave generation.

astro-ph↗

arXiv2001

Enhacement in the dymanic response of a viscoelastic fluid flowing through a longitudinally vibrating tube

We analyzed effects of elasticity on the dynamics of fluids in porous media by studying a flow of a Maxwell fluid in a tube, which oscillates longitudinally and is subject to oscillatory pressure gradient. The present study investigates novelties brought about into the classic Biot's theory of propagation of elastic waves in a fluid-saturated porous solid by inclusion of non-Newtonian effects that are important, for example, for hydrocarbons. Using the time Fourier transform and transforming the problem into the frequency domain, we calculated: (A) the dynamic permeability and (B) the function $F(κ)$ that measures the deviation from Poiseuille flow friction as a function of frequency parameter $κ$. This provides a more complete theory of flow of Maxwell fluid through the longitudinally oscillating cylindrical tube with the oscillating pressure gradient, which has important practical applications. This study has clearly shown transition from dissipative to elastic regime in which sharp enhancements (resonances) of the flow are found.

physics.geo-ph↗

arXiv2001

Non-Newtonian effects in the peristaltic flow of a Maxwell fluid

We analyzed the effect of viscoelasticity on the dynamics of fluids in porous media by studying the flow of a Maxwell fluid in a circular tube, in which the flow is induced by a wave traveling on the tube wall. The present study investigates novelties brought about into the classic peristaltic mechanism by inclusion of non-Newtonian effects that are important, for example, for hydrocarbons. This problem has numerous applications in various branches of science, including stimulation of fluid flow in porous media under the effect of elastic waves. We have found that in the extreme non-Newtonian regime there is a possibility of a fluid flow in the direction {\it opposite} to the propagation of the wave traveling on the tube wall.

physics.flu-dyn↗

arXiv1998

Sgr A^*: A supermassive black hole or a spatially extended object?

We report here on a calculation of possible orbits of the fast moving infrared source S1 which has been recently observed by Eckart and Genzel (1997) near the Galactic center. It is shown that tracking of the orbit of S1 or any other fast moving star near Sgr A^* offers a possibility of distinguishing between the supermassive black hole and extended object scenarios of Sgr A^*. In our calculations we assumed that the extended object at the Galactic center is a non-baryonic ball made of degenerate, self-gravitating heavy neutrino matter, as it has been recently proposed by Tsiklauri & Viollier (1998a,b).

astro-ph↗