V. T. Tikhonchuk

Coupling of shear flow and pressure gradient instabilities

The nonlinear dynamics of a shear flow and its subsequent evolution in the equatorial plane of the inner plasma sheet is studied. A linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin-Helmholtz (KH) and Rayleigh-Taylor instabilities. The hybrid vortex mode grows faster than a KH mode, extracts ambient potential energy, and leads to vortex cells that have a larger spatial extent than a simple KH vortex. In the nonlinear stage, vortices become surge-like and may destroy the shear flow region. The relevance of this model to vortex generation and auroral arc intensifications at the inner edge of the plasma sheet is discussed.

Parallel electric fields in dispersive shear Alfvén waves in the dipolar magnetosphere

Existing theories do not explain large parallel electric fields that are associated with keV electron precipitation in auroral arcs. The MHD electron response results in an electrical conductivity in the low altitude magnetosphere that is two orders of magnitude greater than is required. We suggest a new mechanism for forming parallel electric fields based on a nonlocal electron response to standing shear Alfven waves on dipole magnetic fields. Electron trapping is the primary cause of a significant reduction in the collisionless electron conductivity and consequent enhancement of parallel electric fields in the 1–4 mHz frequency range.

Auroral density fluctuations on dispersive field line resonances

A model is presented that describes the excitation of density perturbations and parallel electric fields by standing shear Alfven waves on dipole fields in Earths magnetosphere. The model includes the effects of electron inertia and gyro-kinetic dispersion, accounting for field-aligned variations of the electron and ion temperatures and the ambient plasma density. In a model dipole magnetosphere, it is found that dispersion and nonlinearity determine the depth, spatial structure, and temporal evolution of large-amplitude density fluctuations near to the polar ionospheres. The characteristics of magnetospheric density cavities and their relationship to auroral luminosity, or field-aligned currents, is discussed in the context of recent satellite and ground based observations.

A model of ultrashort laser pulse absorption in solid targets

A model for ultrashort laser pulse absorption and solid target heating has been developed. It combines a description of laser light absorption in the skin layer with a simple model of plasma heating. Heat wave propagation into the cold target material is the only loss mechanism balancing energy deposition due to absorption. An absorption coefficient is derived from the plasma conductivity and includes a description of physical processes responsible for collisional and collisionless skin layer absorption mechanisms. Comparison with recent femtosecond laser pulse interaction experiment data show good agreement over a wide range of pulse intensities. For laser intensities above 1016 W/cm2 plasma hydrodynamical expansion, which is neglected in our model contributes to a discrepancy between the calculated absorption and experimental data.

Ponderomotive saturation of magnetospheric field line resonances

Compressional Alfven waves in the terrestrial magnetospheric cavity constitute a discrete spectrum of global modes which can resonate with specific components of the continuum spectrum of shear Alfven wave (SAW) field line resonant frequencies. We investigate the effect of the ponderomotive force (PF) of excited standing SAWs on the nonlinear saturation of field line resonances (FLRs). In low β plasmas FLR saturation occurs due to a nonlinear phase slip between the fast mode driver and SAW field. Ponderomotive forces also lead to density depletions at the ionospheres, nonlinear narrowing of the FLR and meridional scales comparable to those embedded within temporally modulated discrete auroral arcs. Observational features relating to these effects are discussed.

Electron kinetic effects in standing shear Alfvén waves in the dipolar magnetosphere

The electron kinetic response to an electric current driven by a standing shear Alfven wave (SAW) is considered for the case of a dipolar geometry. The parallel electric field is found from the electron gyrokinetic equation along with the SAW dispersion coefficient. Electron trapping in the dipolar magnetic field significantly reduces the parallel electric conductivity and in this way increases the amplitude of the parallel electric field and SAW dispersion. It is demonstrated that the two-fluid hydrodynamic equations used by many authors significantly underestimate the electron response and, consequently, the magnitude and location of the parallel electric field under conditions where the electron bounce frequency is larger than the SAW frequency. This is especially important in a plasma with density depressions near the Earth’s polar magnetosphere.

Discrete Auroral Arcs and Nonlinear Dispersive Field Line Resonances

Dispersive effects in field line resonances (FLRs) are discussed in the context of potential structures, parallel currents, and auroral density cavities observed by the FAST satellite. Our model includes the Earths dipole magnetic field, and accounts for electron inertia, electron thermal pressure, finite ion gyroradius effects, and field aligned variations of the plasma density and ambient electron and ion temperatures. For realistic backgound parameters, we show that finite plasma temperature effects determine the dynamics of FLRs and that solitary wave structures evolve out of the resonance region, producing deep density cavities above the polar ionospheres. Results are shown to be in reasonable agreement with ground and satellite observations, with the exception of the magnitude of low altitude electric fields.

Interaction of crossed laser beams with plasmas

The parametric interaction of two crossed collimated laser beams with ion plasma modes has been studied. The underlying process is a density grating that is created by the two laser beams. The Bragg diffraction that is produced enhances forward stimulated Brillouin scattering (SBS) which results in a time dependent energy exchange between the two beams. A diversity of other SBS processes which depend on the symmetry of driving laser beams are also discussed.

Nonlinear standing shear Alfvén waves in the Earth's magnetosphere

We present theory and numerical simulations of strong nonlinear effects in standing shear Alfven waves (SAWs) in the Earth`s magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries. In a low beta plasma it is shown that the ponderomotive force can lead to a large-amplitude SAW spatial harmonic generation due to nonlinear coupling between the SAW and a slow magnetosonic wave. The nonlinear coupling leads to secularly growing frequency shifts, and in the case of driven systems, nonlinear dephasing can lead to saturation of the driven wave fields. The results are discussed on the context of their possible relevance to the theory of standing ionospheric cavity wave modes and field line resonances in the high-latitude magnetosphere.

Saturation of backward stimulated Raman scattering and enhancement of laser light scattering in plasmas

Theoretical and numerical calculations are performed using the system of Zakharov and electromagnetic wave equations, to describe the nonlinear behavior of stimulated Raman scattering (SRS) in a finite homogeneous plasma slab. The enhancement of secondary scattering processes due to the nonlinear SRS saturation is investigated. The parametric decay of the resonantly driven Langmuir wave provides a mechanism which saturates SRS and greatly broadens the Langmuir and ion acoustic wave spectra. These enhanced electrostatic fluctuations scatter the incident electromagnetic radiation. Scaling laws for enhanced Brillouin, forward Raman, and anti‐Stokes forward and backward Raman scattering as well as criteria for their strong enhancement are given. The frequency spectra of enhanced Brillouin scattering shows red‐ and blue‐shifted components, with different amplitudes depending on the plasma density and laser intensity. The numerical results have been compared with experimental data providing new or alternative e...