Grigol Gogoberidze

Spectrum of gravitational radiation from primordial turbulence

Energy injection into the early universe can induce turbulent motions of the primordial plasma, which in turn act as a source for gravitational radiation. Earlier work computed the amplitude and characteristic frequency of the relic gravitational wave background, as a function of the total energy injected and the stirring scale of the turbulence. This paper computes the frequency spectrum of relic gravitational radiation from a turbulent source of the stationary Kolmogoroff form which acts for a given duration, making no other approximations. We also show that the limit of long source wavelengths, commonly employed in aeroacoustic problems, is an excellent approximation. The gravitational waves from cosmological turbulence around the electroweak energy scale will be detectable by future space-based laser interferometers for a substantial range of turbulence parameters.

On the nature of incompressible magnetohydrodynamic turbulence

A novel model of incompressible magnetohydrodynamic turbulence in the presence of a strong external magnetic field is proposed for the explanation of recent numerical results. According to the proposed model, in the presence of the strong external magnetic field, incompressible magnetohydrodynamic turbulence becomes nonlocal in the sense that low-frequency modes cause decorrelation of interacting high-frequency modes from the inertial interval. It is shown that the obtained nonlocal spectrum of the inertial range of incompressible magnetohydrodynamic turbulence represents an anisotropic analogue of Kraichnan’s nonlocal spectrum of hydrodynamic turbulence. Based on the analysis performed in the framework of the weak-coupling approximation, which represents one of the equivalent formulations of the direct interaction approximation, it is shown that incompressible magnetohydrodynamic turbulence could be both local and nonlocal, and therefore anisotropic analogues of both the Kolmogorov and Kraichnan spectra ...

Detectability of gravitational waves from phase transitions

Gravitational waves potentially represent our only direct probe of the universe when it was less than one second old. In particular, first-order phase transitions in the early universe can generate a stochastic background of gravitational waves which may be detectable today. We briefly summarize the physical sources of gravitational radiation from phase transitions and present semianalytic expressions for the resulting gravitational wave spectra from three distinct realistic sources: bubble collisions, turbulent plasma motions, and inverse-cascade helical magnetohydrodynamic turbulence. Using phenomenological parameters to describe phase transition properties, we determine the region of parameter space for which gravitational waves can be detected by the proposed Laser Interferometer Space Antenna. The electroweak phase transition is detectable for a wide range of parameters.

Linear coupling and overreflection phenomena of magnetohydrodynamic waves in smooth shear flows

Special features of magnetohydrodynamic waves linear dynamics in smooth shear flows are studied. Quantitative asymptotic and numerical analysis are performed for wide range of system parameters when basic flow has constant shear of velocity and uniform magnetic field is parallel to the basic flow. The special features consist of magnetohydrodynamic wave mutual transformation and overreflection phenomena. The transformation takes place for arbitrary shear rates and involves all magnetohydrodynamic wave modes. While the overreflection occurs only for slow magnetosonic and Alfven waves at high shear rates. Studied phenomena should be decisive in the elaboration of the self-sustaining model of magnetohydrodynamic turbulence in the shear flows.

FARLEY-BUNEMAN INSTABILITY IN THE SOLAR CHROMOSPHERE

The Farley-Buneman instability (FBI) is studied in the partially ionized plasma of the solar chromosphere taking into account the finite magnetization of the ions and Coulomb collisions. We obtain the threshold value for the relative velocity between ions and electrons necessary for the instability to develop. It is shown that Coulomb collisions play a destabilizing role in the sense that they enable the instability even in the regions where the ion magnetization is larger than unity. By applying these results to chromospheric conditions, we show that the FBI cannot be responsible for the quasi-steady heating of the solar chromosphere. However, we do not exclude the instability development locally in the presence of strong cross-field currents and/or strong small-scale magnetic fields. In such cases, FBI should produce locally small-scale, ~0.1-3 m, density irregularities in the solar chromosphere. These irregularities can cause scintillations of radio waves with similar wave lengths and provide a tool for remote chromospheric sensing.

Gamma Ray Burst Constraints on Ultraviolet Lorentz Invariance Violation

We present a unified general formalism for ultraviolet Lorentz invariance violation (LV) testing through electromagnetic wave propagation, based on both dispersion and rotation measure data. This allows for a direct comparison of the efficacy of different data to constrain LV. As an example we study the signature of LV on the rotation of the polarization plane of γ-rays from gamma ray bursts in a LV model. Here γ-ray polarization data can provide a strong constraint on LV, 13 orders of magnitude more restrictive than a potential constraint from the rotation of the cosmic microwave background polarization proposed by Gamboa, Lopez-Sarrion, and Polychronakos [J. Gamboa, J. Lopez-Sarrion, A.P. Polychronakos, Phys. Lett. B 634 (2006) 471].

Polarized cosmological gravitational waves from primordial helical turbulence

We show that helical turbulence produced during a first-order phase transition generates circularly polarized cosmological gravitational waves (GWs). The characteristic frequency of these GWs for an extreme case of the phase transition model is around 10(-3)-10(-2) Hz with an energy density parameter as high as 10(-12)-10(-11). The possibility of detection is briefly discussed.

Quantifying Shear-induced Wave Transformations in the Solar Wind

The possibility of velocity shear-induced linear transformations of different magnetohydrodynamic waves in the solar wind is studied both analytically and numerically. A quantitative analysis of the wave transformation processes for all possible plasma-β regimes is performed. By applying the obtained criteria for effective wave coupling to the solar wind parameters, we show that velocity shear-induced linear transformations of Alfven waves into magnetoacoustic waves could effectively take place for the relatively low frequency Alfven waves in the energy-containing interval. The obtained results are in a good qualitative agreement with the observed features of density perturbations in the solar wind.

Impact of measurement uncertainties on universal scaling of MHD turbulence

Scaling exponents are the central quantitative prediction of theories of turbulence and in-situ satellite observations of the high Reynolds number solar wind flow have provided an extensive testbed of these. We propose a general, instrument independent method to estimate the uncertainty of velocity field fluctuations. We obtain the systematic shift that this uncertainty introduces into the observed spectral exponent. This shift is essential for the correct interpretation of observed scaling exponents. It is sufficient to explain the contradiction between spectral features of the Elsasser fields observed in the solar wind with both theoretical models and numerical simulations of Magnetohydrodynamic turbulence.

On the origin of the drifting subpulses in radio pulsars

We present a model for the main observational characteristics of the radio emission of pulsars with well-organized drifting subpulses. We propose that drifting subpulses result from the modulation of the radio emission mechanism due to long-wavelength drift waves in the magnetosphere. The drift waves are generated at shorter wavelengths, and their non-linear evolution favours accumulation in a specific azimuthal eigenmode with an integral number, m, of nodes encircling the magnetic pole. The electric field of the drift waves is along the magnetic field lines, and this modulates the distribution for particles and hence the radio emission mechanism. The ratio of the frequency of the eigenmode to the rotation frequency of the star is insensitive to the magnetic field strength and the period of rotation, and is of order unity. The period, P3 ,o fthe drifting subpulses is attributed to the mismatch between this frequency and the nearest harmonic of the rotation frequency of the star. Ke yw ords: polarization ‐ radiation mechanisms: non-thermal ‐ pulsars: general.