Andria D. Rogava

Shear flow induced wave couplings in the solar wind

A sheared background flow in a plasma induces coupling between different MHD wave modes, which results in their mutual transformation with corresponding energy redistribution between the modes. In this way, the energy can be transferred from one wave mode to the other, but energy can also be added to or extracted from the background flow. In the present paper we investigate whether the wave coupling and energy transfer mechanisms can operate under solar wind conditions. It is shown that this is indeed the case. Hence, the long-period waves observed in the solar wind at r > 0.3 AU might be generated by much faster periodic oscillations in the photosphere of the Sun. Other possible consequences for the observable peculiar beat phenomena in the wind and acceleration of the wind particles are also discussed.

On the efficiency of particle acceleration by rotating magnetospheres in AGN

Aims. To investigate the efficiency of centrifugal acceleration of particles as a possible mechanism for the generation of ultra-high γ-ray nonthermal emission from TeV blazars, we study the centrifugal acceleration of electrons by rotating magnetic field lines, for an extended range of inclination angles and determine the maximum Lorentz factors γmax attainable by the electrons via this process. Methods. Two principal limiting mechanisms for the particle acceleration, inverse Compton scattering and breakdown of the beadon-the-wire approximation, are examined. Results. Particles may be centrifugally accelerated up to γmax � 10 8 and the main limiting mechanism for the γmax is the inverse Compton scattering. Conclusions. The energy of centrifugally accelerated particles can be amply sufficient for the generation (via inverse Compton scattering) of the ultra-high energy (up to 20 TeV) gamma emission in TeV blazars.

Acoustic waves in unbounded shear flows

The linear evolution of acoustic waves in a fluid flow with uniform mean density and uniform shear of velocity is investigated. The process of the mean flow energy extraction by the three-dimensional acoustic waves, stimulated by the non-normal character of the linear dynamics in the shear flow, is analyzed. The thorough examination of the dynamics of different physical variables characterizing the wave evolution is presented. The physics of gaining of the shear energy by acoustic waves is described.

Velocity shear generated Alfvén waves in electron–positron plasmas

Linear magnetohydrodynamic (MHD) modes in a cold, nonrelativistic electron–positron plasma shear flow are considered. The general set of differential equations, describing the evolution of perturbations in the framework of the nonmodal approach is derived. It is found, that under certain circumstances, the compressional and shear Alfven perturbations may exhibit large transient growth fueled by the mean kinetic energy of the shear flow. The velocity shear also induces mode coupling, allowing the exchange of energy as well as the possibility of a strong mutual transformation of these modes into each other. The compressional Alfven mode may extract the energy of the mean flow and transfer it to the shear Alfven mode via this coupling. The relevance of these new physical effects to provide a better understanding of the laboratory e+e− plasmas is emphasized. It is speculated that the shear‐induced effects in the electron–positron plasmas could also help solve some astrophysical puzzles (e.g., the generation o...

Centrifugally Driven Relativistic Dynamics on Curved Trajectories

Motion of test particles along rotating curved trajectories is considered. The problem is studied both in the laboratory and the rotating frames of reference. It is assumed that the system rotates with the constant angular velocity ω = const. The solutions are found and analyzed for the case when the form of the trajectory is given by an Archimedes spiral. It is found that particles can reach infinity while they move along these trajectories and the physical interpretation of their behaviour is given. The analogy of this idealized study with the motion of particles along the curved rotating magnetic field lines in the pulsar magnetosphere is pointed out. We discuss further physical development (the conserved total energy case, when ω ≠ const) and astrophysical applications (the acceleration of particles in active galactic nuclei) of this theory.

Escaping Radio Emission from Pulsars: Possible Role of Velocity Shear

It is demonstrated that the velocity shear, intrinsic to the e+e- plasma present in the pulsar magnetosphere, can efficiently convert the nonescaping longitudinal Langmuir waves (produced by some kind of a beam or stream instability) into propagating (escaping) electromagnetic waves. It is suggested that this shear-induced transformation may be the basic mechanism needed for the eventual generation of the observed pulsar radio emission.

Compressible hydromagnetic shear flows with anisotropic thermal pressure: Nonmodal study of waves and instabilities

The evolution of linear magnetohydrodynamic waves and plasma instabilities (firehose and mirror) in a compressible, magnetized plane Couette flow with anisotropic thermal pressure is investigated. In the present study we revealed that the pressure anisotropy brings significant novelty to the effect of coupling and linear reciprocal transformation of the wave modes originally discovered [Chagelishvili, Rogava, and Tsiklauri, Phys. Rev. E 53, 6028 (1996)]. It is found that behavior of the firehose and mirror instabilities is drastically changed due to the presence of shear in the flow. These novel effects are caused by the non-normality of linear dynamics in shear flows and they have been revealed through use of the nonmodal approach.

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.

Velocity shear-induced effects on electrostatic ion perturbations

Linear evolution of electrostatic perturbations in an unmagnetized electron–ion plasma shear flow is studied. New physical effects, arising due to the non-normality of linear dynamics are disclosed. A new class of nonperiodic collective mode with vortical motion of ions, characterized by intense energy exchange with the mean flow, is found. It is also shown that the velocity shear induces extraction of the mean flow energy by ion-sound waves and that during the shear-induced evolution the ion-sound waves turn eventually into ion plasma oscillations.

Investigation of Dynamics of Self-Similarly Evolving Magnetic Clouds

Context. Magnetic clouds (MCs) are “magnetized plasma clouds” moving in the solar wind. MCs transport magnetic flux and helicity away from the Sun. These structures are not stationary but experience temporal evolution. Simplified MC models are usually considered. Aims. We investigate the dynamics of more general, radially expanding MCs. They are considered as cylindrically symmetric magnetic structures with low plasma β. Methods. We adopt both a self-similar approach method and a numerical approach. Results. We demonstrate that the forces are balanced in the considered self-similarly evolving, cylindrically symmetric magnetic structures. Explicit analytical expressions for magnetic field, plasma velocity, density, and pressure within MCs are derived. These solutions are characterized by conserved values of magnetic flux and helicity. We also investigate the dynamics of self-similarly evolving MCs by means of the numerical code “Graale”. In addition, their expansion in a medium of higher density and higher plasma β is studied. It is shown that the physical parameters of the MCs maintain their self-similar character throughout their evolution. Conclusions. After comparing different self-similar and numerical solutions, we are able to conclude that the evolving MCs are quite adequately described by our self-similar solutions – they retain their self-similar, coherent nature for quite a long time and over large distances from the Sun.