O. G. Onishchenko

Mirror instability with finite electron temperature effects

A linear theory of mirror instability accounting for the finite electron temperature effects is developed. Using the standard low-frequency approach to the analysis of this instability but including some kinetic effects, we have derived an expression for the growth rate and analyzed the effects of finite electron temperature and arbitrary electron anisotropy. In comparison with earlier analyses which were limited to isotropic electron distributions, consideration of arbitrary electron anisotropy shows that for sufficiently hot electrons an increased electron temperature enhances the growth rate of the mirror instability.

Linear theory of the mirror instability in non‐Maxwellian space plasmas

[1] A unified theory of the mirror instability in space plasmas is developed. In the standard quasi-hydrodynamic approach, the most general mirror-mode dispersion relation is derived and the growth rate of the mirror instability is obtained in terms of arbitrary electron and ion velocity distribution functions. Finite electron temperature effects and arbitrary electron temperature anisotropies are included. The new dispersion relation allows the treatment of more general space plasma equilibria such as the Dory-Guest-Harris (DGH) or Kennel-Ashour-Abdalla (KA) loss cone equilibria, as well as distributions with power law velocity dependence that are modeled by the family of κ-distributions. Under these conditions, we derive the general kinetic mirror instability growth rate including finite electron temperature effects. As for an example of equilibrium particle distribution, we analyze a large class of κ to suprathermal loss cone distributions in view of application to a variety of space plasmas like the solar wind, magnetosheath, ring current plasma, and the magnetospheres of other planets.

Drift-Alfvén vortices in dusty plasmas

A set of two-fluid equations that governs the nonlinear dynamics of drift-Alfven waves in a multicomponent dusty plasma is derived. It consists of the electron continuity equation, the electron parallel equation of motion, and the divergence of the total plasma current density. In the linear limit, we obtain a dispersion relation that shows the coupling between drift-Alfven and drift convective cells. On the other hand, the stationary solutions of the nonlinear equations can be represented as dipolar vortices. The relevance of our investigation to coherent nonlinear structures in space plasmas is pointed out.

Nonlinear mirror waves in non-Maxwellian space plasmas

A theory of finite-amplitude mirror type waves in non-Maxwellian space plasmas is developed. The collisionless kinetic theory in a guiding center approximation, modified for accounting of the finite ion Larmor radius effects, is used as the starting point. The model equation governing the nonlinear dynamics of mirror waves near instability threshold is derived. In the linear approximation it describes the classical mirror instability that is valid for a wide class of the velocity distribution functions. In the nonlinear regime the mirror waves form solitary structures that have the shape of magnetic holes. The formation of such structures and their nonlinear dynamics has been analyzed both analytically and numerically. It is suggested that the main nonlinear mechanism responsible for mirror instability saturation is associated with modification (flattening) of the shape of the background ion distribution function in the region of small parallel particle velocities. The width of this region is of the order of the particle trapping zone in the mirror hole. Near the mirror instability threshold the saturation arises before its width reaches the ion thermal velocity. The nonlinear mode coupling effects in this approximation are smaller and unable to take control over evolution of the space profile of saturated mirror waves or lead to their magnetic collapse. This results in the appearance of quasi-stable solitary mirror structures having the form of deep magnetic depressions. A phenomenological description of this process is formulated. The relevance of the theoretical results to recent satellite observations is stressed.

Drift mirror instability in space plasmas, 2, Nonzero electron temperature effects

A linear theory of drift mirror instability accounting for the nonzero electron temperature effects is developed. Generalizing our previous approach to the analysis of this instability by accounting for a nonvanishing parallel electric field, we have derived the expressions for the mode frequency and instability growth rate. The origin of the electric field is due to the electron pressure gradient which builds up in a plasma with nonzero electron temperature, because the electrons are dragged by mirror-accelerated protons as they pass from regions of high magnetic flux into those of lower magnetic flux. The electrostatic force drag associated with the parallel electric field provides a substantial reduction of the wave phase velocity and increases the drift mirror instability threshold. It is shown that in a plasma with nonzero electron temperature the drift mirror mode is accompanied by the field-aligned current which varies in phase with the compressional changes in the magnetic field. The transition to the cold electron temperature limit is discussed.

Effects of ion temperature gradients on the formation of drift-Alfvén vortex structures in dusty plasmas

A set of equations describing the nonlinear dynamics of drift-Alfven waves in a dusty plasma accounting for the nonzero ion temperature gradients is derived. It is shown that these new equations yield a solution in the form of two-scale dipolar vortex structures propagating with velocities close to the ion-drift velocity in a narrow cone centered around the direction perpendicular to both the external magnetic field and the plasma gradient directions. The typical scales, characteristic vortex velocities as well as the relevant conditions for their existence are discussed. It is shown that nonzero ion temperature gradients substantially enlarge the range of possible propagation directions and characteristic scales of the vortex structures.

Physical mechanisms for electron mirror and field swelling modes

Ion mirror instability is dominant in planetary and cometary magnetosheaths and other high-beta plasmas where the ions are hotter than the electrons. It is associated with a zero-frequency non-propagating mode with the wave vector nearly perpendicular to the ambient magnetic field. The counterparts of this instability in hot electron plasmas are the field swelling and electron mirror instabilities. A theory for these instabilities was developed more than two decades ago (Basu B and Coppi B 1982 Phys. Rev. Lett. 48 799, 1984 Phys. Fluids 27 1187) within the framework of a fluid model. The connection between the two types of instabilities has been analyzed in (Migliuolo S 1986 J. Geophys. Res. 91 7981). In contrast to these papers, we shall here adopt the standard quasi-hydrodynamic approach that is usually used for the study of mirror instabilities. To analyze the electron mirror and field swelling instabilities, we will only use the perpendicular balance condition and the Liouville theorem. We have found that such a description is easier to understand and gives us increased physical insight into the basic physical features of both these instabilities.

Modification of Kolmogorov spectra of weakly turbulent shear Alfvén waves by dust grains

Decay instabilities and Kolmogorov-type spectra of weakly turbulent shear-Alfven waves in dusty plasmas are analyzed in the limit when the wave dispersion is produced solely by the dust grain density inhomogeneity. It is shown that the reduced equations for weakly nonlinear and dispersive waves possess two conservation laws for the wave energy and generalized enstrophy. It turns out that the weakly turbulent plasma Kolmogorov spectra associated with these conservation laws are nonlocal. It is found that the presence of a dust grain inhomogeneity leads to the formation of an Iroshnikov–Kraichnan type energy spectrum related to the energy conservation law. The possibility of the existence of such spectra in space plasmas is discussed. The specific features of the obtained energy spectra can be used for the identification of dust grains in the Earth’s ionosphere, the solar wind and the interstellar medium using the data collected by magnetometers onboard satellites.

Nonlinear ion-drift waves in a nonuniform plasma with nonzero ion-temperature-gradient effects

It is shown that Grad hydrodynamics can be used for the description of nonlinear ion-drift waves in a low β plasma. The set of nonlinear equations which allows us to describe both the ion polarization drift and the nonzero ion Larmor radius effects is derived. The presence of a nonzero ion temperature gradient induces a corresponding perpendicular thermal flux which in turn substantially modifies the transverse stress tensor. Thus, the description of effects associated with the perpendicular ion polarization drift demands corrections in the magnetic viscosity due to the nonzero thermal flux.

Drift-Alfvén vortices in dusty plasmas with non-zero ion-temperature effects

Reduced two-fluid equations governing the nonlinear dynamics of drift Alfven waves in dusty plasmas with non-zero ion temperature are derived. In the linear limit, we find a dispersion relation that shows the coupling between the ion-drift-Shukla-Varma mode, and electron-drift (magnetostatic) and (inertial or kinetic) Alfven waves due to the finite collisionless electron skin depth or Larmor-radius corrections. In contrast to the case of an electron-ion plasma, when the nonlinear drift Alfven vortices are weakly localized, i.e. decrease at infinity as r -1 the presence of the charged dust grains makes exponential localization possible. The physical meaning of such a localization is connected with the fact that charged massive dust granules provide an additional screening that results in stronger localization of the vortex. In several intermediate-β plasmas with 1 » β » m e /m i (m e,i is the electron or ion mass), the localization length approaches a minimum value when the vortex velocity is of the order of the ion diamagnetic drift. It then reaches the value ρ i δ -1/4 d , where ρ i is the ion Larmor radius and δ d is the ratio of the dust to ion densities multiplied by the dust charge number Z d . In the case of very low plasma pressure. β « m e /m i , the vortex is localized with typical seale (λ e ρ i ) 1/2 δ -1/4 d where λ e is the electron skin depth. Our investigation can thus prediet the velocities of coherent nonlinear structures in space plasmas.