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.     

Effect of small-scale bernstein turbulence on low-frequency plasma waves in the preflare solar chromosphere

The studied region is a part of the current circuit of a magnetic loop in a solar active region in the altitude range of 1400–2500 km above the photosphere. At the earliest stage of development of a flare process, the magnetic field of the loop was assumed to be stationary and uniform in the interval corresponding to weak fields (the so-called deca-hectogauss fields). The conditions for emergence and development of instability of the second harmonic of Bernstein modes in this previously unexamined region were determined. This instability (and low-frequency instabilities emerging later) was assumed to be caused by the sub-Dreicer electric field of the loop, while pair Coulomb collisions were considered to be the major factor hindering its development. The obtained extremely low instability thresholds point to the possibility of subsequent emergence of low-frequency instabilities (and plasma waves corresponding to them) with much higher threshold values against the background of saturated Bernstein turbulence. The frequency of electron scattering by turbulence pulsations in this scenario normally exceeds the frequency of pair Coulomb (primarily ion–electron) collisions. Both the quasistatic sub-Dreicer field in the loop and the weak spatial inhomogeneity of plasma temperature and density were taken into account in the process of derivation and analysis of the dispersion relation for low-frequency waves. It was demonstrated that the solutions of the obtained dispersion relation in the cases of prevalent pair Coulomb collisions and dominant electron momentum losses at pulsations of saturated Bernstein turbulence are morphologically similar and differ only in the boundary values of perturbation parameters. In both cases, these solutions correspond to the two wave families, namely, kinetic Alfven waves and kinetic ion acoustic waves. These waves have their own electric fields and may play the important role in the process of preflare acceleration of energetic electrons.     

Baroclinic instability in differentially rotating stars

A linear analysis of baroclinic instability in a stellar radiation zone with radial differential rotation is performed. The instability sets in at a very small rotation inhomogeneity, ΔΩ ～ 10^?3Ω. There are two families of unstable disturbances corresponding to Rossby waves and internal gravity waves. The instability is dynamical: its growth time is several thousand rotation periods but is short compared to the stellar evolution time. A decrease in thermal conductivity amplifies the instability. Unstable disturbances possess kinetic helicity. Magnetic field generation by the turbulence resulting from the instability is possible.     

A note on the emission mechanism of storm radiation

A new mechanism has been proposed for the continuum and burst components of solar storm radiation by Genkin, Erukhimov, and Levin (1989a, b). In this paper, we point out that while bursts can be explained by the proposed mechanism of scattering on plasma turbulence generated density fluctuations, the continuum cannot be explained by sattering on thermal ion density fluctuations. The reason is, under the same coronal conditions, second harmonic emissions will dominate over the fundamental emission due to scattering on thermal ion density fluctuations in contradiction to observations. We also note that the range of plasma wave densities needed for this mechanism may not be realistic for the case of plasma wave generation due to loss cone instability. It is further argued that coalescence of plasma waves with low-frequency waves still seems to be the plausible mechanism.     

Nonlinear low frequency cut-off of the spectrum of the lower hybrid magnetospheric plasma turbulence

By solving the nonlinear equation of the magnetized plasma in the weak turbulence limit the level of the spectral energy density of the lower hybrid oscillations expanding in the plasma of the Earth's polar magnetosphere, is found. As an approximation the instability which initiates turbulence is considered in a plasma with two interpenetrating beams of nonrelativistic electrons with velocities along the geomagnetic field. The saturation of the instability is due to induced scattering of the oscillations by electrons and ions of the plasma.The spectral distribution of the lower hybrid turbulence has a maximum near the low frequency boundary.     

新磁重联理论

本文研究了磁流体力学与高频等离子体波（ 包括纵横模式） 之间的精巧的相互作用。研究表明,这些等离激元会在电流片内诱发一种阻抗不稳定,并最终导至磁重联,出现爆发性不稳定。在高涨的离声湍动情况下,高温电流片模型必须采用反常电导率,而非库仑电导率。理论估算的结果与观测相一致。因此这种计及等离激元有质动力作用的新磁重联理论,基本上能解释耀斑现象。     

Effect of Induced Toroidal Rotation on Poloidal Rotation and Ion Heat Conductivity of Tokamak Edge Plasmas

The transport processes in edge (collisional) plasmas of tokamaks with smooth profiles of macroscopic plasma parameters and induced poloidal and toroidal plasma flows, are considered. The toroidal and poloidal velocities of particles, the radial electric field and the ion heat flux are derived. It is shown that forces, induced by radio frequency waves, plasma turbulence or neutral beam injection, can be used to control the poloidal and toroidal plasma velocities, as well as ion heat conductivity, in a wide range of these values.     

Gravitational instability in isotropic MHD plasma waves

The effect of compressive viscosity, thermal conductivity and radiative heat-loss functions on the gravitational instability of infinitely extended homogeneous MHD plasma has been investigated. By taking in account these parameters we developed the six-order dispersion relation for magnetohydrodynamic (MHD) waves propagating in a homogeneous and isotropic plasma. The general dispersion relation has been developed from set of linearized basic equations and solved analytically to analyse the conditions of instability and instability of self-gravitating plasma embedded in a constant magnetic field. Our result shows that the presence of viscosity and thermal conductivity in a strong magnetic field substantially modifies the fundamental Jeans criterion of gravitational instability.     

Effect of ion and electron beam on kinetic Alfven wave in an inhomogeneous magnetic field

Kinetic Alfven waves are examined in the presence of electron and ion beam and an inhomogeneous magnetic field with bi-Maxwellian distribution function. The theory of particle aspect analysis is used to evaluate the trajectories of the charged particles. The expressions for the field-aligned currents, perpendicular currents (with respect to B ₀), dispersion relation and growth/damping rate with marginal instability criteria are derived. The effect of electron and ion beam and inhomogeneity of magnetic field are discussed. The results are interpreted for the space plasma parameter appropriate to the auroral acceleration region of the earth’s magnetoplasma.     

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)     

Simulation studies of electron acceleration by ion ring distributions in solar flares

Using a 2 1/2-D fully relativistic electromagnetic particle-in-cell code (PIC) we have investigated a potential electron acceleration mechanism in solar flares. The free energy is provided by ions which have a ring velocity distribution about the magnetic field direction. Ion rings may be produced by perpendicular shocks, which could in turn be generated by the super-Alfvénic motion of magnetic flux tubes emerging from the photosphere or by coronal mass ejections (CMEs). Such ion distributions are known to be unstable to the generation of lower hybrid waves, which have phase velocities in excess of the electron thermal speed parallel to the field and can, therefore, resonantly accelerate electrons in that direction. The simulations show the transfer of perpendicular ion energy to energetic electrons via lower hybrid wave turbulence. With plausible ion ring velocities, the process can account for the observationally inferred fluxes and energies of non-thermal electrons during the impulsive phase of flares. Our results also show electrostatic wave generation close to the plasma frequency: we suggest that this is due to a bump-in-tail instability of the electron distribution.     

Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.     

Effects of low-frequency instability on the Hall conductivity in plasma: Applications to astrophysics and space physics

In the experiment presently described (which is the continuation of our previous work) we studied the effect of low-frequency drift wave instability on Hall conductivity in plasma. Using an external oscillation we can affect the drift wave amplitude (mainly around resonance), and the variation on Hall conductivity is observed. The effect is probably to be attributed to electron trapping by the waves potential. Good agreement between experimental and calculated values of azimuthal drift currents near and away from resonance lead us to believe that the proposed explanation by electron trapping is correct.In addition, the interaction of plasma with the magnetic field is important in a large variety of astrophysical phenomena. A large class of solar and magnetospheric phenomena involve the conversion of stored magnetic energy to thermal and kinetic energy of the plasma with mechanism in which important role have the plasma's conductivity. Accordingly, this experimental work must be considered as a good laboratory simulation to solar plasma devices.     

Currents over the auroral arc

The three-dimensional current system over an enhanced conductivity strip identified with an auroral arc is calculated for the case of the magnetospheric plasma convection across this strip. The strip produces a stationary Alfvén wave which propagates along magnetic field lines and is carried simultaneously by the convecting plasma. The Alfvén wave generation corresponds to an appearance of field-aligned currents over the arc. The three-dimensional current system generated over the arc is studied, taking into account reflection of the waves from the ionosphere of the opposite hemisphere. The correspondence of the theory with the experimental results is found.     

A short wavelength instability in the neutral sheet of the earth's geomagnetic tail

In an earlier paper, Bowers (1973), ion plasma oscillations were found to be unstable in the steady state developed by Cowley (1972) for the neutral sheet in the Earth's geomagnetic tail. In this paper a similar stability analysis is carried out but for a different steady state, suggested by Dungey, with the result that unstable waves with frequencies near the electron plasma frequency are found. In the Dungey steady state the current necessary for magnetic field reversal is carried by plasma originating from both the magnetosheath and the lobes of the tail. This modifies the steady state proposed by Alfvén and subsequently developed by Cowley in which all the current is carried by plasma from the lobes of the tail thereby fixing the cross-tail potential Φ. With magnetosheath plasma present the value of Φ is no longer fixed solely by parameters in the lobes of the tail but the cross-tail electric field is still assumed localised in the dusk region of the sheet as in the Cowley model due to the balance of charge required in the neutral sheet. The value of Φ can be expected to increase as magnetic flux is transported to the tail which inflates and causes flux annihilation because the magneto-sheath plasma in the neutral sheet has insufficient pressure to keep the two lobes of the tail apart. The Vlasov-Maxwell set of equations is perturbed and linearised enabling a critical condition for instability to be found for modes propagating across the tail. Typically, this condition requireseΦ≳KT _m whereT _m is the temperature of magnetosheath electrons. The instability occurs in the presence of cold plasma which hasE×B drifted into the neutral sheet from the lobes of the tail. This contrasts with the usual two stream instability which is stabilised by the cold plasma. Once precipitated the instability may be explosive provided current disruption occurs, for then a further increase in Φ will result which drives a greater range of wave numbers unstable thereby causing even more turbulence and an even larger cross-tail electric field. Because of this behaviour the instability may be a trigger for a substorm.     

Polar magnetic substorms

From the world distribution of geomagnetic disturbance, the connection between the electric current in the ionosphere, the field-aligned current and asymmetric equatorial ringcurrent in the magnetosphere is discussed. The partial ring-current in the afternoon-evening region, whose intensity is closely correlated with the AE-index, usually develops and decays earlier than the symmetric ring-current in the course of magnetic storms. The partial ringcurrent seems to have a direct connection with the positive geomagnetic bay in high latitudes in the evening hours through the ionizing effect of the particles leaking from the partial ringcurrent. The dawn-to-dusk electric field in the magnetospheric tail is transferred to the polar ionosphere, producing there the twin vortex Hall current responsible for polar cap geomagnetic variation. The magnetic effect of the associated Pedersen current in the ionosphere is shown to be small but still worth considering. The electrojet near midnight along the auroral oval is thought to appear when the electric conductivity of the ionosphere is locally increased under the presence of large scale dawn-to-dusk electric field. The occasional appearance of a localized abnormal geomagnetic disturbance with reversed direction near the geomagnetic pole seems to suggest the occasional reversal of electric field near the outer surface of the magnetospheric tail, especially when the interplanetary magnetic field is northward.     

太阳大气中动力学阿尔文波的产生与加热研究进展

动力学阿尔文波是垂直波长接近离子回旋半径或者电子惯性长度的色散阿尔文波.由于波的尺度接近粒子的动力学尺度,动力学阿尔文波在太阳和空间等离子体加热、加速等能化现象中起重要作用.因此,动力学阿尔文波通常被认为是日冕加热的候选者.本研究工作深入、系统地调研了太阳大气中动力学阿尔文波的激发和耗散机制.基于日冕等离子体环境,介绍了几种常见的动力学阿尔文波激发机制:温度各向异性不稳定性、场向电流不稳定性、电子束流不稳定性、密度非均匀不稳定性以及共振模式转换.还介绍了太阳大气中动力学阿尔文波的耗散机制,并讨论了这些耗散机制对黑子加热、冕环加热以及冕羽加热的影响.不仅为认识太阳大气中动力学阿尔文波的驱动机制、动力学演化特征以及波粒相互作用提供合理的理论依据,而且有助于揭示日冕等离子体中能量储存和释放、粒子加热等能化现象的微观物理机制.     

Nonlinear generation of ion loss-cone waves by electrostatic turbulence in the magnetosphere

Effects of plasma turbulence on the stability of electrostatic ion loss-cone waves are examined. The turbulence is assumed to be electrostatic with frequencies near 1.5 times the electron gyrofrequency and the frequencies of the generated waves are below the ion plasma frequency ω_pi>. A nonlinear growth rate of the order of 10^?2ω_pi may be obtained, when the amplitude of the turbulence is 20 mV/m. This is comparable to previously found growth rates of the linear ion loss-cone instability, in a plasma with large pitch angle anisotropy. Bounce averaged pitch angle diffusion coefficients are also presented for different models of the ion loss-cone wave spectrum.     

On the possibility of the development of longitudinal wave instabilities on the background of the small-scale Bernstein turbulence in preflare chromosphere of a solar active region

The process of origination and development of instabilities of the longitudinal waves of two types, namely, low-frequency ion-acoustic and high-frequency (“electronic”) Langmuir waves, in the preflare atmosphere of an active solar region are studied. The area under study is located at the chromospheric part of the flare loop near its footpoint. A weak large-scale electric field of flaring loop is the main source of these instabilities. The velocity of an electronic flow in the preflare plasma is supposed to be much lower than thermal electron velocity. Instability development is considered against the background of small-scale Bernstein wave turbulence, which exists in the preflare plasma and has an extremely low threshold of excitation. The necessary conditions for the instability origination and development, as well as the boundary values of the main plasma and wave perturbation parameters, are calculated.     

On the effect of oblique disturbances on Kelvin-Helmholtz instability at magnetospheric boundary layers and in solar wind

This paper is concerned with the Kelvin-Helmholtz instability in the indissipative plasma with an external magnetic field. A detailed analysis is made of the results known from the approximation of a tangential discontinuity. The finiteness of the interface thickness effect is considered numerically at the arbitrary distribution of the density, velocity and magnetic field vectors inside this shear layer. The influence of plasma compressibility with an arbitrarily varying magnetic field is investigated. The main role of oblique disturbances with respect to the flow rate direction is shown under conditions of a large plasma compressibility. As such perturbations move away from the interface, their amplitude is damped much more slowly than in the case of weak compressibility. However, their wavelength remains, approximately, the same as that of longitudinal waves in the case of incompressibility. The linear approximation suggests the importance of oblique waves in the energetics of the interaction between the shear layer and the outward medium. A comparison is made of the instability period on discontinuities in the solar wind, and at magnetospheric and plasmaspheric boundaries, with the range of geomagnetic pulsations.