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David Tsiklauri

Publications and source records attributed to David Tsiklauri.

At least 37 records · Page 2Linked to original sources
arXiv2007

On parallel electric field generation in transversely inhomogeneous plasmas

The generation of parallel electric fields by the propagation of ion cyclotron waves in the plasma with a transverse density inhomogeneity was studied. It was proven that the minimal model required to reproduce the previous kinetic simulation results of E_{||} generation [Tsiklauri et al 2005, Genot et al 2004] is the two-fluid, cold plasma approximation in the linear regime. By considering the numerical solutions it was also shown that the cause of E_{||} generation is the electron and ion flow separation induced by the transverse density inhomogeneity. We also investigate how E_{||} generation is affected by the mass ratio and found that amplitude attained by E_{||} decreases linearly as inverse of the mass ratio m_i/m_e. For realistic mass ratio of m_i/m_e=1836, such empirical scaling law, within a time corresponding to 3 periods of the driving ion cyclotron wave, is producing E_{||}=14 Vm^{-1} for solar coronal parameters. Increase in mass ratio does not have any effect on final parallel (magnetic field aligned) speed attained by electrons. However, parallel ion velocity decreases linearly with inverse of the mass ratio m_i/m_e. These results can be interpreted as following: (i) ion dynamics plays no role in the E_{||} generation; (ii) E_{||} \propto 1/m_i scaling is caused by the fact that omega_d = 0.3 omega_{ci} \propto 1/m_i is decreasing with the increase of ion mass, and hence the electron fluid can effectively "short-circuit" (recombine with) the slowly oscillating ions, hence producing smaller E_{||}.

astro-ph↗
arXiv2007

A minimal model of parallel electric field generation in a transversely inhomogeneous plasma

We study the generation of parallel electric fields by virtue of propagation of ion cyclotron waves (with frequency 0.3 w_{ci}) in the plasma with a transverse density inhomogeneity. Using two-fluid, cold plasma linearised equations, we show for the first time that E_|| generation can be understood by an analytic equation that couples E_|| to the transverse electric field of the driving ion cyclotron wave. In this simplified model, the generated E_|| amplitude e.g. for plausible solar coronal parameters attains values of 90 V/m for the mass ratio m_i/m_e=262, within a time corresponding to 3 periods of the driving ion cyclotron wave. By considering the numerical solutions we also show that the cause of E_|| generation is electron and ion flow separation (which is not the same as electrostatic charge separation) induced by the transverse density inhomogeneity. The model also correctly reproduces the previous kinetic results in that only electrons are accelerated (along the background magnetic field), while ions do not accelerate substantially. We also investigate how E_|| generation is affected by the mass ratio and found that amplitude attained by E_|| decreases linearly as inverse of the mass ratio m_i/m_e, i.e. E_|| \propto 1/m_i. This result contradicts to the earlier suggestion by Genot et al (1999, 2004) that the cause of E_|| generation is the polarisation drift of the driving wave, which scales as \propto m_i. Also, for realistic mass ratio of m_i/m_e=1836 our empirical scaling law is producing E_||=14 V/m (for solar coronal parameters).

astro-ph↗
arXiv2006

MHD versus kinetic effects in the solar coronal heating: a two stage mechanism

Using Particle-In-Cell simulations i.e. in the kinetic plasma description the discovery of a new mechanism of parallel electric field generation was recently reported. Here we show that the electric field generation parallel to the uniform unperturbed magnetic field can be obtained in a much simpler framework using the ideal magnetohydrodynamics (MHD) description. In ideal MHD the electric field parallel to the uniform unperturbed magnetic field appears due to fast magnetosonic waves which are generated by the interaction of weakly non-linear Alfvén waves with the transverse density inhomogeneity. In the context of the coronal heating problem a new {\it two stage mechanism} of plasma heating is presented by putting emphasis, first, on the generation of parallel electric fields within an {\it ideal MHD} description directly, rather than focusing on the enhanced dissipation mechanisms of the Alfvén waves and, second, dissipation of these parallel electric fields via {\it kinetic} effects. It is shown that for a single Alfvén wave harmonic with frequency $ν= 7$ Hz, and longitudinal wavelength $λ_A = 0.63$ Mm for a putative Alfvén speed of 4328 km s$^{-1}$, the generated parallel electric field could account for 10% of the necessary coronal heating requirement. We conjecture that wide spectrum (10$^{-4}-10^3$ Hz) Alfvén waves, based on the observationally constrained spectrum, could provide the necessary coronal heating requirement. By comparing MHD versus kinetic results we also show that there is a clear indication of the {\it anomalous resistivity} which is 100s of times greater than the classical Braginskii value.

astro-ph↗
arXiv2006

A mechanism for parallel electric field generation in the MHD limit: possible implications for the coronal heating problem in the two stage mechanism

We solve numerically ideal, 2.5D, MHD equations in Cartesian coordinates, with a plasma beta of 0.0001 starting from the equilibrium that mimics a footpoint of a large curvature radius solar coronal loop or a polar region plume. On top of such an equilibrium, a purely Alfvénic, linearly polarised, plane wave is launched. In the context of the coronal heating problem a new two stage mechanism of plasma heating is presented by putting emphasis, first, on the generation of parallel electric fields within an ideal MHD description directly, rather than focusing on the enhanced dissipation mechanisms of the Alfvén waves and, second, dissipation of these parallel electric fields via {\it kinetic} effects. It is shown that a single Alfvén wave harmonic with frequency $ν= 7$ Hz and longitudinal wavelength $λ_A = 0.63$ Mm, for a putative Alfvén speed of 4328 km s$^{-1}$, the generated parallel electric field could account for 10% of the necessary coronal heating requirement. We conjecture that wide spectrum (10$^{-4}-10^3$ Hz) Alfvén waves, based on the observationally constrained spectrum, could provide the necessary coronal heating requirement. The exact amount of energy that could be deposited by such waves through our mechanism of parallel electric field generation can only be calculated once a more complete parametric study is done. Thus, the "theoretical spectrum" of the energy stored in parallel electric fields versus frequency needs to be obtained.

astro-ph↗
arXiv2006

A fresh look on the heating mechanisms of the Solar corona

Recently using Particle-In-Cell simulations i.e. in the kinetic plasma description Tsiklauri et al. and Génot et al. reported on a discovery of a new mechanism of parallel electric field generation, which results in electron acceleration. In this work we show that the parallel (to the uniform unperturbed magnetic field) electric field generation can be obtained in much simpler framework using ideal Magnetohydrodynamic (MHD) description, i.e. without resorting to complicated wave particle interaction effects such as ion polarisation drift and resulting space charge separation which seems to be an ultimate cause of the electron acceleration. Further, in the context of the coronal heating problem a new {\it two stage mechanism} of the plasma heating is presented by putting emphasis, first, on the generation of parallel electric fields within {\it ideal MHD} description directly, rather than focusing on the enhanced dissipation mechanisms of the Alfvén waves and, second, dissipation of these parallel electric fields via {\it kinetic} effects. It is shown that a single Alfvén wave harmonic with frequency ($ν= 7$ Hz), (which has longitudinal wavelength $λ_A = 0.63$ Mm for putative Alfvén speed of 4328 km s$^{-1}$) the generated parallel electric field could account for the 10% of the necessary coronal heating requirement. We conjecture that wide spectrum (10$^{-4}-10^3$ Hz) Alfvén waves, based on observationally constrained spectrum, could provide necessary coronal heating requirement.

astro-ph↗
arXiv2005

The Sun as a MHD generator: application of a new heating mechanism for the coronal loops and closed magnetic structures

We investigate a possibility of heating of the loops and other closed magnetic structures in active regions of the solar corona by the flow of solar wind (plus other flows that may be present) across the magnetic field lines (that are perpendicular to the flow), in a similar manner as a conventional MHD generator works. A simple model is formulated which allows to calculate the typical currents generated by this mechanism. This enables to make a judgement whether the loops in solar active regions can be heated by the proposed mechanism through the associated Ohmic dissipation. It is shown that even if the heating region width is of the order of a few proton Larmor radii (this effectively means that a loop is represented by {\it nano-threads}, by analogy with the nano-flare coronal heating mechanism, with width of a few proton Larmor radii) and the plasma flow speed is 50 km s$^{-1}$ then only about 1% of the heating requirement of coronal loops could be met under this conjecture. Other implications of the model, such as a predicted spatial heat distribution along the loop as well as application of the model to the other places e.g. chromosphere and corotating interaction region structures in the solar wind are discussed.

astro-ph↗
arXiv2005

Particle-In-Cell simulations of circularly polarised Alfvén wave phase mixing: a new mechanism for electron acceleration in collisionless plasmas

In this work we used Particle-In-Cell simulations to study the interaction of circularly polarised Alfvén waves with one dimensional plasma density inhomogeneities transverse to the uniform magnetic field (phase mixing) in collisionless plasmas. In our preliminary work we reported discovery of a new electron acceleration mechanism, in which progressive distortion of the Alfvén wave front, due to the differences in local Alfvén speed, generates an oblique (nearly parallel to the magnetic field) electrostatic field. The latter accelerates electrons through the Landau resonance. Here we report a detailed study of this novel mechanism, including: (i) analysis of broadening of the ion distribution function due to the presence of Alfvén waves and (ii) the generation of compressive perturbations due to both weak non-linearity and plasma density inhomogeneity. The amplitude decay law in the inhomogeneous regions, in the kinetic regime, is demonstrated to be the same as in the MHD approximation described by Heyvaerts and Priest (1983).

astro-ph↗
arXiv2005

Phase mixing of shear Alfven waves as a new mechanism for electron acceleration in collisionless, kinetic plasmas

Particle-In-Cell (kinetic) simulations of shear Alfvén wave interaction with one dimensional, across the uniform magnetic field, density inhomogeneity (phase mixing) in collisionless plasma were performed for the first time. As a result a new electron acceleration mechanism is discovered. Progressive distortion of Alfvén wave front, due to the differences in local Alfvén speed, generates nearly parallel to the magnetic field electrostatic fields, which accelerate electrons via Landau damping. Surprisingly, amplitude decay law in the inhomogeneous regions, in the kinetic regime, is the same as in the MHD approximation described by Heyvaerts and Priest (1983).

astro-ph↗
arXiv2002

Phenomenological model of propagation of the elastic waves in a fluid-saturated porous solid with non-zero boundary slip velocity

Zhu & Granick [Phys. Rev. Lett. 87, 096105 (2001)] have recently experimentally established existence of a boundary slip in a Newtonian liquid. They reported typical values of the slip length of the order of few micro-meters. In this light, the effect of introduction of the boundary slip into the theory of propagation of elastic waves in a fluid-saturated porous medium formulated by Biot is investigated. The new model should allow to fit the experimental seismic data in circumstances when Biot's theory fails, as the introduction of phenomenological dependence of the slip velocity upon frequency, which is based on robust physical arguments, adds an additional degree of freedom to the model. If fact, it predicts higher than the Biot's theory values of attenuation coefficients of the both rotational and dilatational waves in the intermediate frequency domain, which is in qualitative agreement with the experimental data. Therefore, the introduction of the boundary slip yields three-fold benefits: (A) Better agreement of theory with an experimental data since the parametric space of the model is larger (includes effects of boundary slip); (B) Possibility to identify types of porous medium and physical situations where boundary slip is important; (C) Constrain model parameters that are related to the boundary slip.

physics.geo-ph↗
arXiv1999

On the gravitational stability of a compressed slab of gas in the background of weakly interacting massive particles

Linear stability of an isothermal, pressure-bounded, self-gravitating gas slab which is gravitationally coupled with the background weakly interacting massive particles (WIMPs) is investigated. Analytic dispersion relations describing such a configuration are derived. Two novel, distinct oscillatory modes are found. Astrophysical implications of the results are discussed.

astro-ph↗
arXiv1999

Dynamics of the star S0-1 and the nature of the compact dark object at the Galactic center

It has been recently shown (Munyaneza, Tsiklauri and Viollier 1998) that the analysis of the orbits of the fast moving stars close to Sgr A$^{*}$ (Eckart and Genzel 1997) provides a valuable tool to probe the gravitational potential near the Galactic center. As an example, we present here the results on a calculation of possible orbits of the star S0-1 in both, the black hole and degenerate neutrino ball scenarios of the central mass, based on the recent measurements of stellar proper motions at the Galactic center by Ghez et al. 1998. Taking into account the error bars of their analysis, it is shown that within a few years time, the orbit of S0-1 may indeed reveal the nature of the supermassive compact dark object at the Galactic center.

astro-ph↗
arXiv1999

Probing the two temperature paradigm for advection dominated accretion flow: test for the component thermalization time-scale passed

We report here on a calculation of thermalization time-scale of the two temperature advection dominated accretion flow (ADAF) model. It is established that time required to equalize the electron and ion temperatures via electron-ion collisions in the ADAF with plausible physical parameters greatly exceeds age of the Universe, which corroborates validity one of the crucial assumptions of the ADAF model, namely the existence of a hot two temperature plasma. This work is motivated by the recent success (Mahadevan 1998a,b) of ADAF model in explaining the emitted spectrum of Sgr A*.

astro-ph↗
arXiv1998

Heavy neutrino ball as a possible solution to the "blackness problem" of the Galactic center

It has been recently shown (Tsiklauri & Viollier, 1998a) that the matter concentration inferred from observed stellar motion at the galactic center (Eckart & Genzel, 1997, MNRAS, 284, 576 and Genzel et al., 1996, ApJ, 472, 153) is consistent with a supermassive object of $2.5 \times 10^6$ solar masses, composed of self-gravitating, degenerate heavy neutrinos. It has been furthermore suggested (Tsiklauri & Viollier, 1998a) that the neutrino ball scenario may have an advantage that it could possibly explain the so-called "blackness problem" of the galactic center. Here, we present a quantitative investigation of this statement, by calculating the emitted spectrum of Sgr A$^*$ in the framework of standard accretion disk theory.

astro-ph↗
arXiv1998

Dark matter concentration in the galactic center

It is shown that the matter concentration observed through stellar motion at the galactic center (Eckart & Genzel, 1997, MNRAS, 284, 576 and Genzel et al., 1996, ApJ, 472, 153) is consistent with a supermassive object of $2.5 \times 10^6$ solar masses composed of self-gravitating, degenerate heavy neutrinos, as an alternative to the black hole interpretation. According to the observational data, the lower bounds on possible neutrino masses are $m_ν\geq 12.0$ keV$/c^2$ for $g=2$ or $m_ν\geq 14.3$ keV$/c^2$ for $g=1$, where $g$ is the spin degeneracy factor. The advantage of this scenario is that it could naturally explain the low X-ray and gamma ray activity of Sgr A$^*$, i.e. the so called "blackness problem" of the galactic center.

astro-ph↗
arXiv1998

Gravitational phase transition of heavy neutrino matter

In the framework of the Thomas-Fermi model at finite temperature, we show that a cooling nondegenerate gas of massive neutrinos will undergo a phase transition in which quasi-degenerate supermassive neutrino stars are formed through gravitational collapse. For neutrinos in the mass range of 10 to 25 keV these compact dark objects could mimic the role of supermassive black holes that are usually assumed to exist at the centres galaxies. Astrophysical implications and constraints on the neutrino mass are discussed for this scenario.

hep-ph↗