Short-wavelength drift dissipative instability in the presence of field-aligned currents

Sharp density gradients coupled with field-aligned currents can give rise to short wavelength (?15 m) drift waves due to collisional effects in the F-region of the auroral ionosphere. In this wavelength range, ion-ion collisions at altitudes of 300–450 km render the ions unmagnetized and a field-aligned current can drive a drift wave, propagating almost transverse to the magnetic field, unstable due to the resistance in electron parallel motion arising from electron collisions.     

Radio wave heating of astrophysical plasmas

The coherent plasma process such as parametric decay instability (PDI) has been applied to a homogeneous and unmagnetized plasma. These instabilities cause anomalous absorption of strong electromagnetic radiation under specific conditions of energy and momentum conservation and thus cause anomalous heating of the plasma. The maximum plasma temperatures reached are functions of luminosity of the radio radiation and plasma parameters. We believe that these processes may be taking place in many astrophysical objects. Here, the conditions in the sources 3C 273, 3C 48 and Crab Nebula are shown to be conducive to the excitation of PDI. These processes also contribute towards the absorption of 21cm radiation     

The instability of the electromagnetic ordinary modes in counterstreaming and counterrotating plasmas

The instability of a linearly-polarised electromagnetic ordinary mode in counterrotating plasmas and propagating perpendicular to a uniform magnetic field caused by a counterstreaming of electrons along the latter is studied using a cold-plasma model. It is found that: (i) In the presence of either a streaming or a rotation or both, the ordinary-wave propagation is possible even for frequencies less than the plasma frequency; (ii) the Coriolis forces like the applied magnetic field stabilise the ordinary modes.     

Two-stream instability in plasmas with arbitrary orientation of magnetic field

The problem of two-stream instability in plasmas where electrons move through ions with arbitrary orientation of the magnetic field is discussed. Electrostatic and electromagnetic instabilities have both been discussed. It is found that the strength and orientation of the magnetic field both affect the electrostatic waves propagating along the streaming direction to a considerable extent. The electromagnetic instability with a cross-field orientation is associated with a larger range of unstable wavenumber and larger growth rates compared to any other coexisting electrostatic instability.     

On the theory of type III radio bursts in the nonhomogeneous interplanetary space (quasi-linear approximation)

Within the limits of geometrical optics frequency characteristics of perturbations of one-dimensionally non-uniform system “electron beam-solar wind plasma” are investigated in linear approximation on the basis of Maxwell equations closed by the derived constitutive equation. The beam is generated by the active region during solar flares and it appears as a source of type III radio emission in the interplanetary space. The appropriate dispersion equation is solved. Resonance interaction of wave with electron beam appears to happen only in two space points. Such transient (pointwise) mechanism of resonance throws light on one of the basic problems of physics of electron beams generated by solar flares: incomparably more long-term time of their existence compared to the time of existence resulting from the former theoretical estimates of velocity of beam energy loss on radiation within the limits of homogeneous medium. The degree and time of electron beam dissipation were determined in quasi-linear approximation.     

Analysis methods for multi-component wave measurements on board the DEMETER spacecraft

We describe analysis methods to estimate parameters of electromagnetic waves based on the multi-component measurements of the DEMETER spacecraft. Using the fact that the wave magnetic field is perpendicular to the wave vector, the wave normal direction can be estimated by different methods. We use these plane-wave estimates to interpret measurements of the observed wave emissions. For instance, we use the recently developed singular value decomposition (SVD) technique. The results of the plane-wave analysis have an advantage that they often allow a straightforward interpretation. These different methods have been successfully tested with the data of previous spacecraft. All these methods are also implemented in the analysis tools designed for the analysis of the DEMETER wave measurements.We show the first results of these analysis techniques for different types of wave emissions observed on board DEMETER. Obliquely propagating right-hand polarized electromagnetic waves at a few hundreds of Hz are usually connected with a multi-ion mode structure below the local proton cyclotron frequency and with a sharp lower cutoff of left-hand polarized waves, as well as with right-hand polarized waves tunelling below the multi-ion cross-over frequency. Electron and proton whistlers are also very frequently observed on DEMETER. An unusual narrow-band emission at 140 Hz (well below the local proton cyclotron frequency) serves us as another case for a detailed analysis. We find that these waves are right-hand polarized and obliquely propagating.Using this example case, we also present analysis methods to estimate continuous distribution of wave energy density as a function of wave vector directions. These techniques of wave distribution function (WDF) analysis need both wave and particle measurements. In the analyzed case, two different methods of WDF analysis give similar results consistent with the results of the plane-wave techniques. To identify the source region we use the backward ray-tracing method. The wave normal direction obtained by the analysis of multi-component data is used for a simulation of wave propagation from the point of measurement. By this procedure, we obtain an inverse trajectory of the wave ray. We can thus follow the ray path back to the anticipated source region which is in our case located a few degrees of latitude to the South from the spacecraft position.     

Alfvénic drift Kelvin-Helmholtz instability in the presence of an equilibrium electric field

Alfvénic drift Kelvin-Helmholtz instability is discussed in the presence of equilibrium electric and magnetic fields which are perpendicular to each other. A dispersion relation is achieved with the help of WKB approximation and the instability criterion is established. A comparison is made with the previous studies and it is shown that the electric field has a stabilizing or destabilizing effect on the system according to the conditions discussed in the text.     

On the spatial structure and dispersion of slow magnetosonic modes coupled with Alfvén modes in planetary magnetospheres due to field line curvature

The structure of the slow mode coupled with Alfvén mode in the axially symmetric magnetosphere is studied in the paper. Due to the coupling, the slow magnetosonic wave gets dispersion across magnetic shells and becomes not strictly guided. The slow mode is found to be captured between the resonant and cutoff surfaces, where the wave vector radial component goes to infinity and to zero, accordingly. The resonant surface is farther from the Earth than the cutoff surface. The slow mode resonance frequency is much lower than the Alfvén resonance frequency due to small value of the sound velocity near the equator. The maximum of the slow mode amplitude expressed in terms of the parallel magnetic field is concentrated near the equator, but expressed in hydromagnetic terms is concentrated near the ionospheres.     

Modulational instability of dust ion acoustic waves in dusty plasmas with superthermal electrons

The propagation of dust ion acoustic waves is studied in plasmas composed of superthermal distributed electrons and stationary dust particles. The nonlinear Schrödinger equation is derived using the reductive perturbation technique and the modulational instability of dust ion acoustic waves is analyzed. Parametric investigations indicate that the presence of superthermal distributed electrons significantly modify the modulational instability and its growth rate. The effect of particle relative density on the wave characters is also investigated.     

Decay instability of kinetic Alfvén waves in preflare plasmas of the loops in active solar regions

We examine the physical conditions for the origin of the decay instability of kinetic Alfvén waves in loop plasmas at the early flare stages. The synchronism conditions are used to derive a modified expression for the nonlinear growth rate of the process of the decay of the primary kinetic Alfvén wave (KAW) into an ion-acoustic wave and a secondary KAW. The threshold amplitude of the primary KAW is calculated in units of the background magnetic field strength in the chromospheric section of loop current circuit.     

Small-scale Langmuir wave instability in preflare chromosphere of solar active region

Necessary conditions have been investigated for the appearance of instability of high-frequency electron Langmuir waves in plasma of solar chromosphere near the foot-point of loop structure. We have considered the earliest stage of a flare process in solar active region. At the chromospheric part of current circuit of a flare loop such instability can appear and develop as the result of combined action of large-scale electric field, Landau damping and collisional processes in preflare plasma. We have investigated the process of instability development for two possible scenarios: (a) when preflare loop plasma has a classical Coulomb conductivity and (b) when anomalous resistance appears due to saturation of Bernstein turbulence. The growth rates of instability have been obtained and analyzed in detail. It has been assumed in the process of calculation that preflare plasma can be described by the FAL model of the solar atmosphere, which takes into account the process of helium diffusion. It has been shown that Langmuir wave instability can appear in its marginal form in the area under investigation either in the presence of Coulomb conductivity or in the presence of saturated Bernstein turbulence. Existence of instability with the growth rate, which changes its sign, proves the principal possibility of generation of nondamping Langmuir waves with small amplitudes.     

Effects of displacement current on wave dispersion relation and polarization properties in auroral plasmas

In-situ observations from the FREJA magnetospheric research satellite and the Fast Auroral SnapshoT satellite have shown that plasma waves are frequently observed in the auroral plasma,which are believed to be fundamentally important in wave energy dissipation and particle energization.However,the effects of a displacement current on these waves have not been examined.Based on the two-fluid theory,we investigate the dispersion relation and polarization properties of fast,Alfven,and slow modes in the presence of a displacement current,and the effects of the displacement current on these waves are also considered.The results show that the wave frequency,polarization,magnetic helicity and other properties for the fast and Alfven modes are highly sensitive to the normalized Alfven velocity v_A/c,plasma betaβ,and propagation angle θ,while for the slow mode the dependence is minor.In particular,for both fast and Alfven modes,the magnetic helicity is obviously different with and without the displacement current,especially for the Alfven mode with the helicity reversals from right-handed to left-handed when v_A/c increases from 0 to 0.3.The charge-neutral condition of both fast and Alfven modes with frequencies larger than the proton cyclotron frequency is invalid in the presence of the displacement current.Moreover,the presence of the displacement current leads to relatively large magnetic compressibility for the Alfven mode and relatively large electron compressibility for the fast mode.These results can be useful for a comprehensive understanding of the wave properties and the physics of particle energization phenomena in auroral plasmas.     

Universal characteristics of ion-acoustic wave dynamics in magnetized plasmas with emphasis on Tsallis distribution

Using the extended Poincaré-Lighthill-Kuo (PLK) reductive perturbation method, which incorporates the phase-shift variations, it is shown that common features on propagation and head-on collisions of ion-acoustic waves exist for a magnetized plasmas of different inertial-less particle distributions. For instance it is remarked that, the soliton amplitude is always independent of magnetic field strength while strictly depends on its angle regarding the propagation direction. Both types of solitons (compressive or rarefactive) are shown to exist which are defined through the critical angle γ=π/2 or other critical values depending on plasma fractional parameters. These critical plasma parameter values also define the sign of head-on collision phase shift. Furthermore, it is proved that for a given set of plasma parameters there is always a relative angle of propagation regarding to that of the magnetic-field for which the soliton width is maximum. Current findings apply to a wide range of magnetized plasmas including those containing background dust ingredients or two-temperature inertial-less particles and may be used to study laboratory or astrophysical magnetoplasmas.     

Structure and stationarity of quasi-perpendicular shocks: Numerical simulations

This paper presents an overview of numerical simulation studies of fast collisionless shocks and compares these simulation results with observations of the Earth's bow shock and theoretical works. Especially, we review the structure and stationarity of the supercritical quasi-perpendicular shocks. In situ observations indicate that these shocks are generally quasi-stationary whereas full particle simulations as well as hybrid simulations often present a strong nonstationary behavior, a shock self-reformation. The simulation results, along with theoretical and observational works, suggest that the classical models of the quasi-stationary structure generated by reflected protons or by dispersive whistlers are not generally applicable for the supercritical quasi-perpendicular shocks and other phenomena are to be included into the model to ensure the observed quasi-stationarity: The role of a small scale turbulence and shock ripples is investigated. The downstream turbulence and the electron dynamics in the quasi-perpendicular shocks are also discussed.     

Collisional evolution of the asteroid belt

A new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than ∼1 km size are strengthless gravitational aggregates (rubble piles). NEAR found Eros not to be a rubble pile, but a shattered collisional fragment, with a through-going fracture system, and an average of about 20 m regolith cover. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Nevertheless, current understanding of asteroid collisions and size-dependent strength, and the observed distribution of rotation rates versus size, have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? An asteroid size distribution is synthesized from the Minor Planet Center listing and results of the Sloan Digital Sky Survey, an Infrared Space Observatory survey, the Small Main-belt Asteroid Spectroscopic Survey and the Infrared Astronomical Satellite survey. A new picture emerges of asteroid collisional evolution, in which the well-known Dohnanyi result, that the size distribution tends toward a self-similar form with a 2.5-index power law, is overturned because of scale-dependent collision physics. Survival of a basaltic crust on Vesta can be accommodated, together with formation of many exposed metal cores. The lifetimes against destruction are estimated as 3 Gyr at the size of Eros, 10 Gyr at ten times that size, and 40 Gyr at the size of Vesta. Eros as a shattered collisional fragment is not highly unusual. The new picture reveals the new possibility of a transition size in the collisional state, where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions. In this case, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover.     

Kinetic Alfven wave driven by density and magnetic field inhomogeneities in plasmas of finite β

On the basis of an analysis of the instability of drift caused by density and magnetic field inhomogeneities in plasmas with finite β, the effect of the instability on the excitation of kinetic Alfven wave (KAW) is probed. In the kinetic theory, which correctly treats the effect of the finite Larmor radius and the wave-particle resonant interaction, the motion of the ions is described with the Vlasov equation and the motion of electrons, with the kinetic drift equation. Comparing the effects by inhomogeneities in the density and in the magnetic field in plasmas with finite β, we found that the drift instability is more easily excited by the former, and in the instability so excited, the energy transfer is more intense. This energy transfer provides the physical basis for the excitation of KAW. As shown by numerical solutions, KAWs can be widely excited and produced in the magnetosphere, especially in the cusp of the magnetosphere, in the magnetopause and in the boundary layers of plasma sheets, where inhomogeneities are obvious. The results of the present work further illustrate that the KAW plays an important role in the energy transfer in magnetospheric regions.     

Alfvén wave heating and acceleration of plasmas in the solar transition region producing jet-like eruptive activity

We present an analysis of observations and theory of selected transition-region phenomena, concentrating on small scale jet-like structures known as spicules and macrospicules. We examine a number of mechanisms that may be responsible for their formation and conclude that Alfvén waves could provide the necessary acceleration through the ponderomotive force and dissipation for heating forming a beam or jet like structure. In applying the Alfvén wave model we make no fundamental distinction between spicules and macrospicules. In this respect we consider them to be manifestations of the same phenomenon on different scales. We predict that the most effective Alfvén waves have frequencies around 1 Hz and amplitudes of 1 V m^–1. The resulting plasma jet sets up plasma conditions suitable for creating rotating structures which are also observed.