Ion Acoustic Turbulence Research Articles

Amplification of ion-acoustic turbulence upon electron-cyclotron heating of plasma

In this paper, we present the results of the study of ion-acoustic turbulence upon electron-cyclotron heating of low-temperature plasma in a TAU-1 model setup. Two MI-167 magnetrons were used as microwave sources. The peak power of each magnetron was 1 kW, the pulse duration was 7.5 µs. An increase in the temperature of most electrons and an increase in the energy of nonthermal electrons were observed in experiments. Microwave field turning-on caused an increase by an order of magnitude in the spectral density of ion-acoustic noise in the entire frequency range from 0.3 to 3 MHz under study.

Decay of beam-driven Langmuir wave into ion-acoustic turbulence in two dimensions

The decay of beam-generated Langmuir wave into another Langmuir wave and an ion-acoustic wave is a well-known problem with wide-ranging applications. However, most discussions in the literature are based upon simple one-dimensional approximation. Recently, the present authors carried out a fully self-consistent two-dimensional analysis of the beam-driven Langmuir wave decay problem. The main focus of the present authors' work to date had been on the nonlinear evolution of Langmuir turbulence and its influence on the electrons. Relatively little attention had been paid to the ion-acoustic wave generation. In the present discussion, the focus is placed on the dynamics of ion-acoustic turbulence that results from the decay of beam-generated Langmuir wave. The present analysis considers three electron components, the dense core, a primary beam and a counter-streaming beam. We find that the ion-sound turbulence level sensitively depends on the properties of the counter-streaming beam.

Reflection and absorption of HF radiation by a turbulent plasma

Reflection and absorption of a linearly polarized wave by a plasma with developed ion-acoustic turbulence is studied. The dependences of the absorption coefficient and Stokes parameters of the reflected elliptically polarized wave on the anisotropic effective frequency of electron collisions are established.

Proton heating by parallel Alfvén wave cascade

In a recent series of papers, the present authors developed a kinetic theory for low-frequency turbulence propagating parallel to the ambient magnetic field. Making use of this theory, it was shown that low-frequency Alfvénic turbulence may cascade to ion-cyclotron frequency range and beyond by nonlinear three-wave decay processes. The significance of such a finding is that it may lead to the proton heating by cyclotron resonance. However, the actual proton heating process was not demonstrated. The present paper complements the previous works by including the proton heating in the discussion. It is found that the left-hand circularly polarized Alfvén-cyclotron turbulence leads to a moderate heating of the protons in the perpendicular direction and cooling in the parallel direction. It is also found that ion-acoustic turbulence is generated by the decay instability process. Finally, the heating rate is shown to increase in inverse proportion to the time scale of the wave source.

THERMAL FRONTS IN FLARING MAGNETIC LOOPS

We examine the existence of thermal fronts in solar flaring magnetic loops. In the past, two opposite conclusions have been made from the analysis of the evolution of a system in which hot electrons are injected into a coronal magnetic loop. One, made on the basis of analytical solutions, claims that this leads to the formation of ion-acoustic turbulence and subsequently a thermal front develops, while the opposing view obtained later on the basis of detailed numerical modeling shows no evidence of thermal fronts. In our study a one dimensional Vlasov solver is implemented to solve the distribution functions for each particle species fa(r, v, t). The numerical simulations are electrostatic since we examine motions along the field lines only. The results presented here show that earlier numerical results only showed the absence of thermal fronts because of the restricted size of the initial hot-electron region. If larger hot regions are simulated then thermal fronts do indeed form. This dependence on the system size is also explained. These results therefore confirm some of the theory presented by Levin and Melnikov and Brown et al. (i.e., the hot electrons are confined). The net effect of thermal fronts is that the electron energy is confined (mi/me)1/2 times longer than the estimate based on the free streaming of hot electrons.

Cross-Scale Effects in Solar-Wind Turbulence

The understanding of the small-scale termination of the turbulent energy cascade in collisionless plasmas is nowadays one of the outstanding problems in space physics. In the absence of collisional viscosity, the dynamics at small scales is presumably kinetic in nature; the identification of the physical mechanism which replaces energy dissipation and establishes the link between macroscopic and microscopic scales would open a new scenario in the study of turbulent heating in space plasmas. We present a numerical analysis of kinetic effects along the turbulent energy cascade in solar-wind plasmas which provides an effective unified interpretation of a wide set of spacecraft observations and shows that, simultaneously with an increase in the ion perpendicular temperature, strong bursts of electrostatic activity in the form of ion-acoustic turbulence are produced together with accelerated beams in the ion distribution function.

Influence of the anomalous thermal conductivity on the structure of the solar atmosphere transition region

The generalized Wiedemann-Franz law for a nonisothermal quasi-neutral plasma with developed ion-acoustic turbulence and Coulomb collisions has been proven. The results obtained are used to explain the anomalously low thermal conductivity in the chromosphere-corona transition region of the solar atmosphere. Model temperature distributions in the lower corona and the transition region that correspond to well-known experimental data have been determined. The results obtained are useful for explaining the abrupt change in turbulent-plasma temperature at distances smaller than the particle mean free path.

Generalized Wiedemann–Franz law and temperature jump in space plasmas with ion-acoustic turbulence

Abstract In this Letter, the generalized Wiedemann–Franz law for the non-isothermic quasi-neutral plasma with developed ion-acoustic turbulence is proved. This law determines the relationship between electrical and thermal conductivities in a plasma with well-developed ion-acoustic turbulence. The anomalously low thermal conductivity, which leads to the formation of high temperature gradients in the zone of the temperature jump, is explained. Results are used to explain some properties of the solar chromosphere–corona transition region.

On the “injection problem” at the solar wind termination shock

This article presents an integrated analytical model on the injection efficiencies of the different ion species of the Anomalous component of the Cosmic Rays (ACRs) at the solar wind termination shock. We find that the injection into diffusive (first-order Fermi) acceleration is dominated by parallel ion diffusion and not by perpendicular diffusion unless the angle Ψ between the shock normal and the heliospheric magnetic field is almost exactly 90° (). In steady state the threshold speed for injection into first-order Fermi acceleration at a not exactly perpendicular solar wind termination shock – with the Parker shock angle – adjusts itself self-consistently. Increased anisotropic ACR flux amplifies Alfvénic turbulence which in turn suppresses parallel diffusion. It therefore increases the injection threshold and decreases the ACR flux until equilibrium is reached. For this equilibrium situation, we estimate the injection efficiencies of different species of suprathermal ions at the termination shock. We consider the following pre-acceleration processes: 1) momentum diffusion in compressional (ion-acoustic and magnetosonic) turbulence in the upstream supersonic solar wind and adiabatic cooling during convection to the termination shock; 2) reflection, transmission, and acceleration in the electric potential of the termination shock; and 3) momentum diffusion (stochastic or second-order Fermi acceleration) in the subsonic solar wind downstream of the termination shock in the inner heliosheath region. Our model results are compared to data from instruments on board the SOHO, ACE, Ulysses, and Voyager spacecraft.

The Influence of Ion-Acoustic Turbulence on the Electron Acceleration in the Reconnecting Current Sheet

Through solving the single electron equation of motion and the Fokker-Planck equation including the terms of electric field strength and ion-acoustic turbulence, we study the influence of the ion-acoustic wave on the electron acceleration in turbulent reconnecting current sheets. It is shown that the ion-acoustic turbulence which causes plasma heating rather than particle acceleration should be considered. With typical parameter values, the acceleration time scale is around the order of 10(-6) s, the accelerated electrons may have approximately a power-law distribution in the energy range 20 similar to 100 keV and the spectral index is about 3 similar to 10, which is basically consistent with,the observed hard X-ray spectra in solar flares.

Kinetic Alfvén waves in preflare plasma

Low-frequency instabilities of plasma waves in the arch structures in solar active regions have been investigated before a flare. In the framework of mechanism of “direct initiation” of instability by slowly increasing (quasi-static) large-scale electric field in a loop the dispersion relation has been studied for the perturbations which propagate almost perpendicularly to the magnetic field of the loop. The case has been considered, when amplitude of weak (“subdreicer”) electric field sharply increases before a flare, low-frequency instability develops on the background of ion-acoustic turbulence and thickness of this turbulent plasma layer plays the role of mean characteristic scale of inhomogeneity of plasma density. If the values of the main plasma parameters, i.e. temperature, density, magnetic field amplitude allow to neglect the influence of the shear of magnetic strength lines on the instability development, then two types of the waves can be generated in preflare plasma: the kinetic Alfvén waves and some new kind of the waves from the range of slowly magneto-acoustic ones. Instability of kinetic Alfvén waves has clearly expressed threshold character with respect to the amplitude of “subdreicer” electric field. This fact seems to be useful for the short-time prediction of a flare in arch structure. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Particle-in-cell simulations of heat flux driven ion acoustic instability

The return current instability of ion acoustic waves in a laser heated plasma is studied by means of a collisional particle-in-cell code and theoretical analysis in the regime of nonlocal heat transport. The physical scenario of localized, inverse Bremsstrahlung heating in a single laser hot spot, electron thermal transport, return current of cold electrons, instability of ion acoustic waves, and resulting ion acoustic turbulence are examined in a self-consistent kinetic collisional particle simulation. The observed growth of the return current instability is in excellent agreement with predictions of a linear, nonlocal theory. Ion acoustic fluctuations contribute to the inhibition of thermal transport, which leads to the enhancement of the electron temperature in the center of a hot spot. Increased electron collisionality and hot ion tail production are the dominant saturation mechanisms of the return current instability in a one-dimensional geometry. The effects of the ion acoustic turbulence on other interaction processes are also discussed.

Recent progress in nonlinear kinetic Alfvén waves

Abstract. This paper presents a review of recent progress in nonlinear kinetic Alfvén wave (KAW hereafter). We start with the two-fluid theory of KAWs and show how the difference between the motions of electrons and ions in small-scale fields of KAWs modifies the Alfvén wave properties. Then, we focus on nonlinear solitary structures of KAWs. A general criterion of the existence for solitary KAW (SKAW hereafter) and its exact analytical solution in a low-β plasma (β<me/mi are presented, where the electron drift velocity along the background magnetic field is larger than the thermal speed within a SKAW, and hence can excite, for instance, ion acoustic turbulence as showed by in situ observations of satellites in space plasmas. In consequence, the turbulence results in kinetic dissipation of the SKAW and dynamical evolution in its structure. We further discuss the structure of the dissipated SKAW (DSKAW hereafter) that evolves from the SKAW due to the dissipation. The result shows that the DSKAW has a local shock-like structure in its density profile and a net electric potential drop over the shock-like structure. In particular, the electric potential drop of the DSKAW can be expected to accelerate electrons efficiently to the order of the local Alfvén speed. The application of the DSKAW acceleration mechanism to the auroral electron acceleration is also discussed. Finally, a few perspectives of KAW studies in future are presented.

Ion velocity distribution in a plasma with low-intensity ion acoustic turbulence

The quasi-steady ion distribution in a plasma with a single ion species and with low-intensity ion acoustic turbulence is found. Conditions are determined under which the stimulated scattering of ion acoustic waves by ions leads to the formation of a superthermal ion distribution function that decreases with increasing velocity more gradually than does a Maxwellian distribution function. It is found that the plasma conductivity increases as a result of a decrease in the turbulence level due to an enhancement of the Cherenkov damping of ion acoustic waves by resonant ions, whose number increases because of the formation of a gradually decreasing distribution of superthermal ions.

Electric Field, Magnetic Field, and Density Measurements on the Active Plasma Experiment Sounding Rocket

High-resolution, in situ measurements of dc and wave electric fields, magnetic fields, and plasma number density have been gathered by instruments on a diagnostic payload at which a high-velocity, overdense aluminum ion beam was directed from a separate payload spaced 468 m away. The experiment, called the Active Plasma Physics Experiment, was carried out in the Earth's high-latitude ionosphere at 360-km altitude using a sounding rocket. The experimental data clearly show a large diamagnetic cavity with a 93 % depletion of the Earth's magnetic field within a narrowly confined ( 1.5 V/m perpendicular to the magnetic field were observed that represented both the E × B bulk plasma velocity and a magnetosonic wave, which preceded the arrival of the beam that was also evident in the AB magnetometer data. The electric field data also show the presence of electric fields parallel to the magnetic field, including a bipolar electric field signature presumably set up to ensure current continuity. Other plasma waves associated with the release include Alfven perturbations, intense broadband turbulence extending to frequencies beyond 1 MHz, whistler-mode electromagnetic emissions at the ambient O + lower hybrid frequency, and ion acoustic turbulence. The measurements provide a self-consistent picture of the electrodynamics surrounding a high-velocity, overdense ion beam released in the high-latitude ionosphere.

Nonlinear frequency shift of the dust ion-acoustic wave

The nonlinear frequency shift of a low-frequency, coherent dust ion-acoustic wave in the presence of higher-frequency dust ion-acoustic turbulence is investigated in the framework of weak turbulence theory. It is found that the frequency shift of the dust ion-acoustic wave in an unmagnetized dusty plasma is always positive irrespective of the propagation direction of the coherent wave, as in the case of the customary ion-acoustic wave in the absence of dust particles. It is also found that the magnitude of the frequency shift decreases with increasing dust density.

Turbulent ion heating in a plasma with two ion species

A description of ion heating by ion-acoustic turbulence in a plasma containing two ion species of different charge-to-mass ratio is presented. It is shown that, when collisions between ions of different species are ignored, only one of the ion species is heated substantially while the temperature of the other species approaches a constant value that differs insignificantly from the initial ion temperature. The only exception to this is a case in which the plasma parameters are in a particular relationship. For an example of a fully ionized HO plasma, it is shown that taking into account the collisional energy exchange between different ion species removes this restriction on the ion temperature.

Properties of fast magnetosonic shocklets at the bow shock

We present detailed properties of fast magnetosonic shocklets (1000 km in size and amplitude of 10 times the background B‐field values) observed in natural unbounded collisionless plasma by a fleet of four Cluster spacecraft at the quasi‐parallel bow shock. We provide for the first time details of the internal structure of shocklets which have magnetic pulses shifted in time from the plasma density pulses, have localized regions with ion acoustic turbulence, and are shown to slow down and thermalize the solar wind ions. The shocklets emit weak electron plasma waves which are used to verify the plasma density profiles within the structure. They move with supersonic speeds (∼250 km/s) and appear in the region with mixing of cold solar wind ion beams and thermalized heated plasma. We provide a qualitative explanation for the observed relationship between the magnetic field and plasma density profiles within the structure based on a nonlinear momentum conservation equation of plasma fluid dynamics.

Dynamic polarizability of plasma and turbulent heating of light and heavy ions

The turbulent heating of the bulk ions is studied for plasmas with light and heavy ions. The role of the dynamic polarizability of plasma in the nonlinear theory of ion-acoustic turbulence is established for these plasmas. It is shown that the account for the dynamic polarizability gives a faster heating of heavy ions and a slower heating of light ions.

Global extended magnetohydrodynamic studies of fast magnetic reconnection

Recent experimental and theoretical results have led to two lines of thought regarding the physical processes underlying fast magnetic reconnection. One is based on the traditional Sweet–Parker model but replaces the Spitzer resistivity with an enhanced resistivity caused by electron scattering by ion acoustic turbulence. The other includes the finite gyroradius effects that enter Ohm’s law through the Hall and electron pressure gradient terms. A two-dimensional numerical study, conducted with a new implicit parallel two-fluid code, has helped to clarify the similarities and differences in predictions between these two models. The former yields resistivity-dependent reconnection with a thick, moderate-aspect-ratio current sheet. If the sheet thickness is less than or comparable to the ion skin depth, it is verified that the Hall effect will predominate [Shay et al., Geophys. Res. Lett. 26, 2163 (1999)], producing true fast reconnection with a microscopic current sheet of unit aspect ratio and a distorted out-of-plane magnetic field [Mandt et al., Geophys. Res. Lett. 21, 73 (1994)].