Author name code: voitenko ADS astronomy entries on 2022-09-14 author:"Voitenko, Yuriy M." ------------------------------------------------------------------------ Title: Initiation of Alfvénic turbulence by Alfvén wave collisions: A numerical study Authors: Shestov, S. V.; Voitenko, Y. M.; Zhukov, A. N. Bibcode: 2022A&A...661A..93S Altcode: 2022arXiv220308643S In the framework of compressional magnetohydrodynamics (MHD), we numerically studied the commonly accepted presumption that the Alfvénic turbulence is generated by the collisions between counter-propagating Alfvén waves (AWs). In the conditions typical for the low-beta solar corona and inner solar wind, we launched two counter-propagating AWs in the three-dimensional simulation box and analyzed polarization and spectral properties of perturbations generated before and after AW collisions. The observed post-collisional perturbations have different polarizations and smaller cross-field scales than the original waves, which supports theoretical scenarios with direct turbulent cascades. However, contrary to theoretical expectations, the spectral transport is strongly suppressed at the scales satisfying the classic critical balance of incompressional MHD. Instead, a modified critical balance can be established by colliding AWs with significantly shorter perpendicular scales. We discuss consequences of these effects for the turbulence dynamics and turbulent heating of compressional plasmas. In particular, solar coronal loops can be heated by the strong turbulent cascade if the characteristic widths of the loop substructures are more than ten times smaller than the loop width. The revealed new properties of AW collisions have to be incorporated in the theoretical models of AW turbulence and related applications. Title: Quantifying Wave-Particle Interactions in Collisionless Plasmas: Theory and Its Application to the Alfvén-mode Wave Authors: Zhao, Jinsong; Lee, Louchuang; Xie, Huasheng; Yao, Yuhang; Wu, Dejin; Voitenko, Yuriy; Pierrard, Viviane, Bibcode: 2022ApJ...930...95Z Altcode: Wave-particle interactions can induce energy transfer at different timescales in collisionless plasmas, which leads to the reshaping of the particle velocity distribution function. Therefore, how to quantify wave-particle interactions is one of the fundamental problems in the heliosphere and in astrophysical plasmas. This study proposes a systematic method to quantify linear wave-particle interactions based on the Vlasov-Maxwellian model. We introduce energy transfer rates with various expressions by using perturbed electric fields and perturbed particle velocity distribution functions. Then, we use different expressions of the energy transfer rate to perform a comprehensive investigation of wave-particle interactions of the Alfvén-mode wave. We clarify the physical mechanisms responsible for the damping of the Alfvén-mode wave in wavevector space. Moreover, this study exhibits for the first time evident signatures of wave-particle interactions between Alfvén-mode waves and resonant/nonresonant particles in the velocity space. These resonant and nonresonant particles can induce energy transfer in opposite directions, which leads to self-regulation of the particle velocity distribution function. Furthermore, this study exhibits a comprehensive dependence of wave-particle interactions of the Alfvén-mode wave on the wavenumber and plasma beta (the ratio between the plasma thermal pressure and the magnetic pressure). These results illustrate that the proposed method would be very useful for quantifying different types of linear wave-particle interactions of an arbitrary wave mode. Title: Electron Heat Flux Instabilities in the Inner Heliosphere: Radial Distribution and Implication on the Evolution of the Electron Velocity Distribution Function Authors: Sun, Heyu; Zhao, Jinsong; Liu, Wen; Voitenko, Yuriy; Pierrard, Viviane; Shi, Chen; Yao, Yuhang; Xie, Huasheng; Wu, Dejin Bibcode: 2021ApJ...916L...4S Altcode: This Letter investigates the electron heat flux instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. Our results show that both the electron acoustic wave and the oblique whistler wave are unstable in the regime with large relative drift speed (ΔVe) between electron beam and core populations. Landau-resonant interactions of electron acoustic waves increase the electron parallel temperature that would lead to suppressing the electron acoustic instability and amplifying the growth of oblique whistler waves. Therefore, we propose that the electron heat flux can effectively drive oblique whistler waves in an anisotropic electron velocity distribution function. This study also finds that lower-hybrid waves and oblique Alfvén waves can be triggered in the solar atmosphere, and that the former instability is much stronger than the latter. Moreover, we clarify that the excitation of lower-hybrid waves mainly results from the transit-time interaction of beaming electrons with resonant velocities v∥ ~ ω/k∥, where ω and k∥ are the wave frequency and parallel wavenumber, respectively. In addition, this study shows that the instability of quasi-parallel whistler waves can dominate the regime with medium ΔVe at the heliocentric distance nearly larger than 10 times of the solar radius. Title: Model of imbalanced kinetic Alfvén turbulence with energy exchange between dominant and subdominant components Authors: Gogoberidze, G.; Voitenko, Y. M. Bibcode: 2020MNRAS.497.3472G Altcode: 2020MNRAS.tmp.2256G Alfvénic turbulence in the fast solar wind is imbalanced: the energy of the (dominant) waves propagating outward from the Sun is much larger than energy of inward-propagating (subdominant) waves. At large scales Alfvén waves are non-dispersive and turbulence is driven by non-linear interactions of counter-propagating waves. Contrary to this, at kinetic scales Alfvén waves become dispersive and non-linear interactions become possible among co-propagating waves as well. The study of the transition between these two regimes of Alfvénic turbulence is important for understanding of complicated dynamics of imbalanced Alfvénic turbulence. In this paper, we present a semiphenomenological model of the imbalanced Alfvénic turbulence accounting for the energy exchange between the dominant and subdominant wave fractions. The energy transfer becomes non-negligible at sufficiently small yet still larger than the ion gyroradius scales and is driven by the non-linear beatings between dispersive dominant(subdominant) waves pumping energy into the subdominant(dominant) component. Our results demonstrate that the turbulence imbalance should decrease significantly in the weakly dispersive wavenumber range. Title: Spectrum of imbalanced Alfvénic turbulence at ion-kinetic scales in the solar wind Authors: Gogoberidze, G.; Voitenko, Y. M. Bibcode: 2020Ap&SS.365..149G Altcode: Using a semi-phenomenological model of the imbalanced Alfvénic turbulence, we study the turbulence transition from magnetohydrodynamic to sub-ion kinetic scales in the conditions typical for the fast solar wind. We show that the transition takes place in a wide wave number interval where the energy spectrum does not follow any power law. For larger turbulence imbalances, both boundaries of the transition wave number interval shift to larger scales. The energy exchange between dominant and subdominant components of the turbulence is essential in the model reducing the imbalance at kinetic scales; however, the total balance is never reached. Our results are compatible with recent solar wind observations at ion scales and numerical simulations of imbalanced turbulence. Title: Temperature spectra in the solar wind turbulence Authors: Gogoberidze, G.; Voitenko, Y. M.; Machabeli, G. Bibcode: 2018MNRAS.480.1864G Altcode: 2018MNRAS.tmp.1821G We study Alfvénic turbulent fluctuations and their spectral properties from MHD to kinetic scales and compare with recent measurements of the Spektr-R spacecraft. An apparent contradiction is found between the temperature spectra derived from the Spektr-R data and the temperature spectra predicted theoretically. To resolve this contradiction, we show that the temperature fluctuations can be correctly estimated from the Spektr-R data only if the mean temperature is isotropic. Since the mean temperature in the solar wind is usually anisotropic, the derived fluctuations appear to be pseudo-temperature rather than temperature. These pseudo-temperature fluctuations are driven by the high-amplitude magnetic fluctuations in Alfvén waves rather than the fluctuations of temperature or thermal velocity. That is why their amplitudes are usually significantly larger than the amplitudes of authentic temperature fluctuations. Title: Non-resonant Alfvénic instability activated by high temperature of ion beams in compensated-current astrophysical plasmas Authors: Malovichko, P.; Voitenko, Y.; De Keyser, J. Bibcode: 2018A&A...615A.169M Altcode: 2018arXiv180402599M Context. Compensated-current systems are established in response to hot ion beams in terrestrial foreshock regions, around supernova remnants, and in other space and astrophysical plasmas.
Aims: We study a non-resonant reactive instability of Alfvén waves propagating quasi-parallel to the background magnetic field B0 in such systems.
Methods: The instability is investigated analytically in the framework of kinetic theory applied to the hydrogen plasmas penetrated by hot proton beams.
Results: The instability arises at parallel wavenumbers kz that are sufficiently large to demagnetize the beam ions, kzVTb/ωBi ≳ 1 (here VTb is the beam thermal speed along B0 and ωBi is the ion-cyclotron frequency). The Alfvén mode is then made unstable by the imbalance of perturbed currents carried by the magnetized background electrons and partially demagnetized beam ions. The destabilizing effects of the beam temperature and the temperature dependence of the instability threshold and growth rate are demonstrated for the first time. The beam temperature, density, and bulk speed are all destabilizing and can be combined in a single destabilizing factor αb triggering the instability at αb > αbthr, where the threshold value varies in a narrow range 2.43 ≤ αbthr ≤ 4.87. New analytical expressions for the instability growth rate and its boundary in the parameter space are obtained and can be directly compared with observations. Two applications to terrestrial foreshocks and foreshocks around supernova remnants are briefly discussed. In particular, our results suggest that the ions reflected by the shocks around supernova remnants can drive stronger instability than the cosmic rays. Title: Nonlinear Decay of Alfvén Waves Driven by Interplaying Two- and Three-dimensional Nonlinear Interactions Authors: Zhao, J. S.; Voitenko, Y.; De Keyser, J.; Wu, D. J. Bibcode: 2018ApJ...857...42Z Altcode: We study the decay of Alfvén waves in the solar wind, accounting for the joint operation of two-dimensional (2D) scalar and three-dimensional (3D) vector nonlinear interactions between Alfvén and slow waves. These interactions have previously been studied separately in long- and short-wavelength limits where they lead to 2D scalar and 3D vector decays, correspondingly. The joined action of the scalar and vector interactions shifts the transition between 2D and 3D decays to significantly smaller wavenumbers than was predicted by Zhao et al. who compared separate scalar and vector decays. In application to the broadband Alfvén waves in the solar wind, this means that the vector nonlinear coupling dominates in the extended wavenumber range 5 × 10-4 ≲ ρ i k 0⊥ ≲ 1, where the decay is essentially 3D and nonlocal, generating product Alfvén and slow waves around the ion gyroscale. Here ρ i is the ion gyroradius, and k 0⊥ is the pump Alfvén wavenumber. It appears that, except for the smallest wavenumbers at and below {ρ }i{k}0\perp ∼ {10}-4 in Channel I, the nonlinear decay of magnetohydrodynamic Alfvén waves propagating from the Sun is nonlocal and cannot generate counter-propagating Alfvén waves with similar scales needed for the turbulent cascade. Evaluation of the nonlinear frequency shift shows that product Alfvén waves can still be approximately described as normal Alfvénic eigenmodes. On the contrary, nonlinearly driven slow waves deviate considerably from normal modes and are therefore difficult to identify on the basis of their phase velocities and/or polarization. Title: Solar Plasma Radio Emission in the Presence of Imbalanced Turbulence of Kinetic-Scale Alfvén Waves Authors: Lyubchyk, O.; Kontar, E. P.; Voitenko, Y. M.; Bian, N. H.; Melrose, D. B. Bibcode: 2017SoPh..292..117L Altcode: 2017arXiv170702295L We study the influence of kinetic-scale Alfvénic turbulence on the generation of plasma radio emission in the solar coronal regions where the ratio β of plasma to magnetic pressure is lower than the electron-to-ion mass ratio me/mi. The present study is motivated by the phenomenon of solar type I radio storms that are associated with the strong magnetic field of active regions. The measured brightness temperature of the type I storms can be up to 1010K for continuum emission, and can exceed 1011K for type I bursts. At present, there is no generally accepted theory explaining such high brightness temperatures and some other properties of the type I storms. We propose a model with an imbalanced turbulence of kinetic-scale Alfvén waves that produce an asymmetric quasi-linear plateau on the upper half of the electron velocity distribution. The Landau damping of resonant Langmuir waves is suppressed and their amplitudes grow spontaneously above the thermal level. The estimated saturation level of Langmuir waves is high enough to generate observed type I radio emission at the fundamental plasma frequency. Harmonic emission does not appear in our model because the backward-propagating Langmuir waves undergo strong Landau damping. Our model predicts 100 % polarization in the sense of the ordinary (o-) mode of type I emission. Title: MHD-Kinetic Transition in Imbalanced Alfvénic Turbulence Authors: Voitenko, Yuriy; De Keyser, Johan Bibcode: 2016ApJ...832L..20V Altcode: 2016arXiv160907173V Alfvénic turbulence in space is usually imbalanced: amplitudes of waves propagating parallel and anti-parallel to the mean magnetic field {{\boldsymbol{B}}}0 are unequal. It is commonly accepted that the turbulence is driven by (counter-)collisions between these counter-propagating wave fractions. Contrary to this, we found a new ion-scale dynamical range of the turbulence established by (co-)collisions among waves co-propagating in the same direction along {{\boldsymbol{B}}}0. Co-collisions become stronger than counter-collisions and produce steep non-universal spectra above certain wavenumbers dependent on the imbalance. Spectral indexes of the strong turbulence vary around ≳ -3, such that steeper spectra follow larger imbalances. Intermittency steepens the -3 spectra further, up to -3.7. Our theoretical predictions are compatible with steep variable spectra observed in the solar wind at ion kinetic scales, but further verifications are needed by correlating observed spectra with measured imbalances. Title: Imbalanced magnetohydrodynamic turbulence modified by velocity shear in the solar wind Authors: Gogoberidze, G.; Voitenko, Y. M. Bibcode: 2016Ap&SS.361..364G Altcode: We study incompressible imbalanced magnetohydrodynamic turbulence in the presence of background velocity shears. Using scaling arguments, we show that the turbulent cascade is significantly accelerated when the background velocity shear is stronger than the velocity shears in the subdominant Alfvén waves at the injection scale. The spectral transport is then controlled by the background shear rather than the turbulent shears and the Tchen spectrum with spectral index -1 is formed. This spectrum extends from the injection scale to the scale of the spectral break where the subdominant wave shear becomes equal to the background shear. The estimated spectral breaks and power spectra are in good agreement with those observed in the fast solar wind. The proposed mechanism can contribute to enhanced turbulent cascades and modified -1 spectra observed in the fast solar wind with strong velocity shears. This mechanism can also operate in many other astrophysical environments where turbulence develops on top of non-uniform plasma flows. Title: Imbalanced magnetohydrodynamic turbulence modified by velocity shear in the solar wind Authors: Gogoberidze, Grigol; Voitenko, Yuriy Bibcode: 2016arXiv161007073G Altcode: We study incompressible imbalanced magnetohydrodynamic turbulence in the presence of background velocity shears. Using scaling arguments, we show that the turbulent cascade is significantly accelerated when the background velocity shear is stronger than the velocity shears in the subdominant Alfv% én waves at the injection scale. The spectral transport is then controlled by the background shear rather than the turbulent shears and the Tchen spectrum with spectral index $-1$ is formed. This spectrum extends from the injection scale to the scale of the spectral break where the subdominant wave shear becomes equal to the background shear. The estimated spectral breaks and power spectra are in good agreement with those observed in the fast solar wind. The proposed mechanism can contribute to enhanced turbulent cascades and modified $-1$ spectra observed in the fast solar wind with strong velocity shears. This mechanism can also operate in many other astrophysical environments where turbulence develops on top of non-uniform plasma flows. Title: Kinetic Alfvén turbulence below and above ion cyclotron frequency Authors: Zhao, J. S.; Voitenko, Y. M.; Wu, D. J.; Yu, M. Y. Bibcode: 2016JGRA..121....5Z Altcode: 2015arXiv150908237Z Alfvénic turbulent cascade perpendicular and parallel to the background magnetic field is studied accounting for anisotropic dispersive effects and turbulent intermittency. The perpendicular dispersion and intermittency make the perpendicular-wave-number magnetic spectra steeper and speed up production of high ion cyclotron frequencies by the turbulent cascade. On the contrary, the parallel dispersion makes the spectra flatter and decelerate the frequency cascade above the ion cyclotron frequency. Competition of these factors results in spectral indices distributed in the interval [-2, -3], where -2 is the index of high-frequency space-filling turbulence and -3 is the index of low-frequency intermittent turbulence formed by tube-like fluctuations. Spectra of fully intermittent turbulence fill a narrower range of spectral indices [-7/3, -3], which almost coincides with the range of indexes measured in the solar wind. This suggests that the kinetic-scale turbulent spectra are mainly shaped by the dispersion and intermittency. A small mismatch with measured indexes of about 0.1 can be associated with damping effects not studied here. Title: Compensated-current instability of kinetic Alfvén waves Authors: Malovichko, P.; Voitenko, Y.; De Keyser, J. Bibcode: 2015MNRAS.452.4236M Altcode: We study a non-resonant instability of kinetic Alfvén waves (KAWs) driven by compensated currents. Such currents set up in response to energetic ion beams occurring in many space and astrophysical plasmas, like foreshock regions in the solar wind and around supernova remnants. Kinetic effects of the background ion gyroradius make the KAW instability stronger than its magnetohydrodynamic (MHD) counterpart and shift its maximum to shorter wavelengths. The KAW growth time can be very short, approaching the proton gyroperiod in the terrestrial foreshock ahead of the quasi-perpendicular bow shock region. The oblique Alfvén instability driven by the cosmic rays in the interstellar and intergalactic plasmas develops mostly in the MHD regime and can extend in the KAW regime only at large fluxes of cosmic rays. Short cross-field wavelengths of growing Alfvén modes facilitate stochastic cross-field acceleration of cosmic rays. Title: Nonlinear Damping of Alfvén Waves in the Solar Corona Below 1.5 Solar Radii Authors: Zhao, J. S.; Voitenko, Y.; Guo, Y.; Su, J. T.; Wu, D. J. Bibcode: 2015ApJ...811...88Z Altcode: Nonthermal velocities measured in the solar corona imply a strong damping of upward-propagating low-frequency ≲ 0.01 {Hz} Alfvén waves at heliocentric distances from 1.02 to 1.4 solar radii. We propose a vector Alfvén wave decay as a feasible mechanism for the observed Alfvén wave damping. Contrary to the extensively studied scalar decay, the vector decay does not depend on the wave frequency and can be efficient for low-frequency coronal Alfvén waves. We show that the vector decay is much stronger than the scalar decay and can provide the observed damping of 0.01 Hz coronal Alfvén waves with perpendicular wavelengths of ∼ {10}4 {km} or less. Fully three-dimensional (3D) numerical simulations are needed to capture this decay, whose growth rate is proportional to the vector product of interacting wave vectors. Title: Generation of Proton Beams by Non-uniform Solar Wind Turbulence Authors: Voitenko, Y.; Pierrard, V. Bibcode: 2015SoPh..290.1231V Altcode: 2015SoPh..tmp...20V Recent theoretical advances and observations indicate that magneto-hydrodynamic (MHD) Alfvénic turbulence is anisotropic and cascades mainly toward small scales perpendicular to the mean magnetic field. Eventually, the turbulence cascade reaches the ion-gyroradius scales where Alfvénic turbulent fluctuations possess parallel electric fields. We show that the local enhancements of the solar-wind turbulence can generate proton beams running ahead of these enhancements. The basic process leading to the beam formation is proton reflections off the turbulent fluctuations at the MHD/kinetic spectral break. With the turbulence amplitudes observed in the solar wind, theory predicts beam number densities of about 0.1 of the background number density and beam velocities of about 1.3 of the Alfvén velocity. These values fit the beam parameters measured in the solar wind well. In general, the more energetic proton beams with higher densities and velocities originate from higher turbulence levels and/or a hotter proton background. The higher the spectral break wavenumber, the faster the generated proton beam. These trends are to be examined by future solar wind observations. Title: Scalar and Vector Nonlinear Decays of Low-frequency Alfvén Waves Authors: Zhao, J. S.; Voitenko, Y.; De Keyser, J.; Wu, D. J. Bibcode: 2015ApJ...799..222Z Altcode: We found several efficient nonlinear decays for Alfvén waves in the solar wind conditions. Depending on the wavelength, the dominant decay is controlled by the nonlinearities proportional to either scalar or vector products of wavevectors. The two-mode decays of the pump MHD Alfvén wave into co- and counter-propagating product Alfvén and slow waves are controlled by the scalar nonlinearities at long wavelengths ρ i2k0\perp 2<ω 0/ω ci (k 0 is wavenumber perpendicular to the background magnetic field, ω0 is frequency of the pump Alfvén wave, ρ i is ion gyroradius, and ω ci is ion-cyclotron frequency). The scalar decays exhibit both local and nonlocal properties and can generate not only MHD-scale but also kinetic-scale Alfvén and slow waves, which can strongly accelerate spectral transport. All waves in the scalar decays propagate in the same plane, hence these decays are two-dimensional. At shorter wavelengths, ρ i2k0\perp 2\gtω 0/ω ci, three-dimensional vector decays dominate generating out-of-plane product waves. The two-mode decays dominate from MHD up to ion scales ρ i k 0 ~= 0.3; at shorter scales the one-mode vector decays become stronger and generate only Alfvén product waves. In the solar wind the two-mode decays have high growth rates >0.1ω0 and can explain the origin of slow waves observed at kinetic scales. Title: Solar wind acceleration obtained from kinetic models based on electron velocity distribution functions with suprathermal particles Authors: Pierrard, V.; Pieters, M.; Lazar, M.; Voitenko, Y.; Lamy, H.; Echim, M. Bibcode: 2014AGUFMSM51B4258P Altcode: Astrophysical and space plasmas are commonly found to be out ofthermal equilibrium, i.e., the velocity distribution functions (VDF)of plasma particles cannot be described well enough by Maxwelliandistribution functions. The suprathermal populations are ubiquitousenhancing the high-energy tail of the distribution. A kinetic model has been developed to successfullydescribe such plasmas with tails decreasing as a power law of thevelocity. In the present work, we show that a natural heating ofsolar and stellar coronas automatically appears when an enhancedpopulation of suprathermal particles is present at low altitude inthe solar (or stellar) atmosphere. This is true not only forelectrons and protons, but also for the minor ions which exhibit atemperature increase proportional to their mass. Moreover,suprathermal electrons contribute to the acceleration of stellarwinds to high bulk velocities when Coulomb collisions are neglected.These results are illustrated by using a global model of the solarcorona and solar wind based on VDF with suprathermal tails for thedifferent particle species. The energetic particles are non-collisional (without Coulomb collisions) even when thermalparticles are submitted to collisions. In the presence of long-rangecorrelations supplied by the fields and plasma instabilities,turbulence can play a role in the generation of such suprathermaltails. Solar wind observations are used as boundary conditions to determine the VDF in the other regions of the heliosphere. Consequences of suprathermal particles are also illustratedfor other space plasmas like the plasmasphere and the polar wind ofthe Earth and other planets. Title: Properties of Short-wavelength Oblique Alfvén and Slow Waves Authors: Zhao, J. S.; Voitenko, Y.; Yu, M. Y.; Lu, J. Y.; Wu, D. J. Bibcode: 2014ApJ...793..107Z Altcode: 2014arXiv1405.0717Z Linear properties of kinetic Alfvén waves (KAWs) and kinetic slow waves (KSWs) are studied in the framework of two-fluid magnetohydrodynamics. We obtain the wave dispersion relations that are valid in a wide range of the wave frequency ω and plasma-to-magnetic pressure ratio β. The KAW frequency can reach and exceed the ion-cyclotron frequency at ion kinetic scales, whereas the KSW frequency remains sub-cyclotron. At β ~ 1, the plasma and magnetic pressure perturbations of both modes are in anti-phase, so that there is nearly no total pressure perturbations. However, these modes also exhibit several opposite properties. At high β, the electric polarization ratios of KAWs and KSWs are opposite at the ion gyroradius scale, where KAWs are polarized in the sense of electron gyration (right-hand polarized) and KSWs are left-hand polarized. The magnetic helicity σ ~ 1 for KAWs and σ ~ -1 for KSWs, and the ion Alfvén ratio RAi Lt 1 for KAWs and RAi Gt 1 for KSWs. We also found transition wavenumbers where KAWs change their polarization from left-handed to right-handed. These new properties can be used to discriminate KAWs and KSWs when interpreting kinetic-scale electromagnetic fluctuations observed in various solar-terrestrial plasmas. This concerns, in particular, identification of modes responsible for kinetic-scale pressure-balanced fluctuations and turbulence in the solar wind. Title: Nonlinear Generation of Kinetic-scale Waves by Magnetohydrodynamic Alfvén Waves and Nonlocal Spectral Transport in the Solar Wind Authors: Zhao, J. S.; Voitenko, Y.; Wu, D. J.; De Keyser, J. Bibcode: 2014ApJ...785..139Z Altcode: We study the nonlocal nonlinear coupling and generation of kinetic Alfvén waves (KAWs) and kinetic slow waves (KSWs) by magnetohydrodynamic Alfvén waves (MHD AWs) in conditions typical for the solar wind in the inner heliosphere. This cross-scale process provides an alternative to the turbulent energy cascade passing through many intermediate scales. The nonlinearities we study are proportional to the scalar products of wave vectors and hence are called "scalar" ones. Despite the strong Landau damping of kinetic waves, we found fast growing KAWs and KSWs at perpendicular wavelengths close to the ion gyroradius. Using the parametric decay formalism, we investigate two independent decay channels for the pump AW: forward decay (involving co-propagating product waves) and backward decay (involving counter-propagating product waves). The growth rate of the forward decay is typically 0.05 but can exceed 0.1 of the pump wave frequency. The resulting spectral transport is nonlocal and anisotropic, sharply increasing perpendicular wavenumbers but not parallel ones. AWs and KAWs propagating against the pump AW grow with about the same rate and contribute to the sunward wave flux in the solar wind. Our results suggest that the nonlocal decay of MHD AWs into KAWs and KSWs is a robust mechanism for the cross-scale spectral transport of the wave energy from MHD to dissipative kinetic scales in the solar wind and similar media. Title: Electrostatic plasma instabilities driven by neutral gas flows in the solar chromosphere Authors: Gogoberidze, G.; Voitenko, Y.; Poedts, S.; De Keyser, J. Bibcode: 2014MNRAS.438.3568G Altcode: 2013arXiv1312.5767G; 2014MNRAS.tmp..148G We investigate electrostatic plasma instabilities of Farley-Buneman (FB) type driven by quasi-stationary neutral gas flows in the solar chromosphere. The role of these instabilities in the chromosphere is clarified. We find that the destabilizing ion thermal effect is highly reduced by the Coulomb collisions and can be ignored for the chromospheric FB-type instabilities. In contrast, the destabilizing electron thermal effect is important and causes a significant reduction of the neutral drag velocity triggering the instability. The resulting threshold velocity is found as function of chromospheric height. Our results indicate that the FB-type instabilities are still less efficient in the global chromospheric heating than the Joule dissipation of the currents driving these instabilities. This conclusion does not exclude the possibility that the FB-type instabilities develop in the places where the cross-field currents overcome the threshold value and contribute to the heating locally. Typical length-scales of plasma density fluctuations produced by these instabilities are determined by the wavelengths of unstable modes, which are in the range 10-102 cm in the lower chromosphere and 102-103 cm in the upper chromosphere. These results suggest that the decimetric radio waves undergoing scattering (scintillations) by these plasma irregularities can serve as a tool for remote probing of the solar chromosphere at different heights. Title: Oblique Alfvén Instabilities Driven by Compensated Currents Authors: Malovichko, P.; Voitenko, Y.; De Keyser, J. Bibcode: 2014ApJ...780..175M Altcode: 2013arXiv1312.7358M Compensated-current systems created by energetic ion beams are widespread in space and astrophysical plasmas. The well-known examples are foreshock regions in the solar wind and around supernova remnants. We found a new oblique Alfvénic instability driven by compensated currents flowing along the background magnetic field. Because of the vastly different electron and ion gyroradii, oblique Alfvénic perturbations react differently on the currents carried by the hot ion beams and the return electron currents. Ultimately, this difference leads to a non-resonant aperiodic instability at perpendicular wavelengths close to the beam ion gyroradius. The instability growth rate increases with increasing beam current and temperature. In the solar wind upstream of Earth's bow shock, the instability growth time can drop below 10 proton cyclotron periods. Our results suggest that this instability can contribute to the turbulence and ion acceleration in space and astrophysical foreshocks. Title: Velocity-Space Proton Diffusion in the Solar Wind Turbulence Authors: Voitenko, Y.; Pierrard, V. Bibcode: 2013SoPh..288..369V Altcode: 2013arXiv1304.1200V We study the velocity-space quasi-linear diffusion of the solar wind protons driven by oblique Alfvén turbulence at proton kinetic scales. Turbulent fluctuations at these scales possess the properties of kinetic Alfvén waves (KAWs) that are efficient in Cherenkov-resonant interactions. The proton diffusion proceeds via Cherenkov kicks and forms a quasi-linear plateau - the nonthermal proton tail in the velocity distribution function (VDF). The tails extend in velocity space along the mean magnetic field from 1 to (1.5 - 3) VA, depending on the spectral break position, on the turbulence amplitude at the spectral break, and on the spectral slope after the break. The most favorable conditions for the tail generation occur in the regions where the proton thermal and Alfvén velocities are about equal, VTp/VA≈1. The estimated formation times are within 1 - 2 h for typical tails at 1 AU, which is much shorter than the solar wind expansion time. Our results suggest that the nonthermal proton tails, observed in situ at all heliocentric distances > 0.3 AU, are formed locally in the solar wind by the KAW turbulence. We also suggest that the bump-on-tail features - proton beams, often seen in the proton VDFs, can be formed at a later evolutional stage of the nonthermal tails by the time-of-flight effects. Title: Modification of Proton Velocity Distributions by Alfvénic Turbulence in the Solar Wind Authors: Pierrard, V.; Voitenko, Y. Bibcode: 2013SoPh..288..355P Altcode: 2013arXiv1304.2154P In the present paper, the proton velocity distribution function (VDF) in the solar wind is determined by numerically solving the kinetic evolution equation. We compare the results obtained when considering the effects of external forces and Coulomb collisions with those obtained by adding effects of Alfvén wave turbulence. We use Fokker-Planck diffusion terms to calculate the Alfvénic turbulence, which take into account observed turbulence spectra and kinetic effects of the finite proton gyroradius. Assuming a displaced Maxwellian for the proton VDF at the simulation boundary at 14 solar radii, we show that the turbulence leads to a fast (within several solar radii) development of the anti-sunward tail in the proton VDF. Our results provide a natural explanation for the nonthermal tails in the proton VDFs, which are often observed in-situ in the solar wind beyond 0.3 AU. Title: Turbulent spectra and spectral kinks in the transition range from MHD to kinetic Alfvén turbulence Authors: Voitenko, Y.; de Keyser, J. Bibcode: 2011NPGeo..18..587V Altcode: 2011arXiv1105.1941V A weakly dispersive range (WDR) of kinetic Alfvén turbulence is identified and investigated for the first time in the context of the MHD/kinetic turbulence transition. We find perpendicular wavenumber spectra ∝ k\bot-3 and ∝ k\bot-4 formed in WDR by strong and weak turbulence of kinetic Alfvén waves (KAWs), respectively. These steep WDR spectra connect shallower spectra in the MHD and strongly dispersive KAW ranges, which results in a specific double-kink (2-k) pattern often seen in observed turbulent spectra. The first kink occurs where MHD turbulence transforms into weakly dispersive KAW turbulence; the second one is between weakly and strongly dispersive KAW ranges. Our analysis suggests that partial turbulence dissipation due to amplitude-dependent non-adiabatic ion heating may occur in the vicinity of the first spectral kink. The threshold-like nature of this process results in a conditional selective dissipation that affects only the largest over-threshold amplitudes and that decreases the intermittency in the range below the first spectral kink. Several recent counter-intuitive observational findings can be explained by the coupling between such a selective dissipation and the nonlinear interaction among weakly dispersive KAWs. Title: Velocity Distributions and Proton Beam Production in the Solar Wind Authors: Pierrard, Viviane; Voitenko, Yuriy Bibcode: 2010AIPC.1216..102P Altcode: Helios, Ulysses, and Wind spacecraft have observed the velocity distribution functions (VDFs) of solar wind particles deviating significantly from Maxwellians. We review recent models using different approximations and mechanisms that determine various observed characteristics of the VDFs for the electrons, protons and minor ions. A new generation mechanism is proposed for super-Alfvénic proton beams and tails that are often observed in the fast solar wind. The mechanism is based on the proton trapping and acceleration by kinetic Alfvén waves (KAWs), which carry a field-aligned potential well propagating with super-Alfvén velocities. Title: Torsional Alfvén waves in small scale current threads of the solar corona Authors: Copil, P.; Voitenko, Y.; Goossens, M. Bibcode: 2010A&A...510A..17C Altcode: Context. The magnetic field structuring in the solar corona occurs on large scales (loops and funnels), but also on small scales. For instance, coronal loops are made up of thin strands with different densities and magnetic fields across the loop.
Aims: We consider a thin current thread and model it as a magnetic flux tube with twisted magnetic field inside the tube and straight field outside. We prove the existence of trapped Alfvén modes in twisted magnetic flux tubes (current threads) and we calculate the wave profile in the radial direction for two different magnetic twist models.
Methods: We used the Hall MHD equations that we linearized in order to derive and solve the eigenmode equation for the torsional Alfvén waves.
Results: We show that the trapped Alfv én eigenmodes do exist and are localized in thin current threads where the magnetic field is twisted. The wave spectrum is discrete in phase velocity, and the number of modes is finite and depends on the amount of the magnetic field twist. The phase speeds of the modes are between the minimum of the Alfvén speed in the interior and the exterior Alfén speed.
Conclusions: Torsional Alfvén waves can be guided by thin twisted magnetic flux-tubes (current threads) in the solar corona. We suggest that the current threads guiding torsional Alfvén waves, are subject to enhanced plasma heating due to wave dissipation. Title: Kinetic Alfven instabilities and anomalous resistivity in inhomogenous current sheets Authors: Voitenko, Yuriy; de Keyser, Johan Bibcode: 2010cosp...38.1937V Altcode: 2010cosp.meet.1937V As it has been shown recently, the current-driven instability (CDI) of kinetic Alfven waves (KAWs) has a lower threshold current density than other known instabilities in low-beta uni-form plasmas. The currents flowing between interacting magnetic fluxes are concentrated in thin current sheets and are thus very inhomogeneous, which makes waves and instabilities in such sheets much different from those in uniform plasmas. We show that the current inhomo-geneity has a profound influence on KAWs and their CDI. The KAW phase velocity spans a wide velocity range decreasing from super-Alfvenic velocities to zero with increasing current shear. In this velocity range KAWs undergo a kinetic instability that depends on both desta-bilizing factors: current strength and current shear. The kinetic instability has a low threshold and can attain a high growth rate with moderate current shears. For stronger current shears, the KAW phase velocity becomes imaginary and the KAW transforms into a purely growing mode (aperiodic instability). Both instabilities can develop in the same current sheet simulta-neously but in different regions: aperiodic instability at the flanks of the current sheet where the current shear maximizes, and the kinetic instability shifted towards current sheet center, where the current density is higher. The anomalous resistivity generated by the near-threshold regime of the inhomogeneous CDI of KAWs is capable of supporting a fast (Petschek) magnetic reconnection. The corresponding current sheet width is about 20 gyroradii, which is 3 times thicker then in the case of homogenous CDI. It seems that the interplay of kinetic and purely growing KAW instabilities can make magnetic reconnection intrinsically intermittent. Possible implications of our results are discussed in the context of magnetic reconnection at the Earth's magnetopause and in the solar corona. Title: Farley-Buneman Instability in the Solar Chromosphere Authors: Gogoberidze, G.; Voitenko, Y.; Poedts, S.; Goossens, M. Bibcode: 2009ApJ...706L..12G Altcode: 2009arXiv0902.4426G The Farley-Buneman instability (FBI) is studied in the partially ionized plasma of the solar chromosphere taking into account the finite magnetization of the ions and Coulomb collisions. We obtain the threshold value for the relative velocity between ions and electrons necessary for the instability to develop. It is shown that Coulomb collisions play a destabilizing role in the sense that they enable the instability even in the regions where the ion magnetization is larger than unity. By applying these results to chromospheric conditions, we show that the FBI cannot be responsible for the quasi-steady heating of the solar chromosphere. However, we do not exclude the instability development locally in the presence of strong cross-field currents and/or strong small-scale magnetic fields. In such cases, FBI should produce locally small-scale, ~0.1-3 m, density irregularities in the solar chromosphere. These irregularities can cause scintillations of radio waves with similar wave lengths and provide a tool for remote chromospheric sensing. Title: Torsional Alfvén waves in small scale density threads of the solar corona Authors: Copil, P.; Voitenko, Y.; Goossens, M. Bibcode: 2008A&A...478..921C Altcode: The density structuring of the solar corona is observed at large scales (loops and funnels), but also at small scales (sub-structures of loops and funnels). Coronal loops consist of thin density threads with sizes down to (and most probably below) the resolution limit. We study properties of torsional Alfvén waves propagating in inhomogeneous cylindrical density threads using the two-fluid magnetohydrodynamic equations. The eigenmode solutions supported by such a structure are obtained and analysed. It is shown that the dispersive and dissipative effects become important for the waves localised in thin threads. In this case, the Alfvén wave continuum is replaced with a discrete spectrum of Alfvén waves. This mathematical model is applied to the waves propagating in coronal structures. In particular, we consider ~1 Hz Alfvén waves propagating along density threads with a relatively smooth radial profile, where a density contrast of about 1.1 is attained at radial distances of about 0.1 km. We found that the dissipation distance of these waves is less than the typical length of hot coronal loops, 50 Mm. Torsional Alfvén waves are localised in thin density threads and produce localised heating. Therefore, these waves can be responsible for coronal heating and for maintenance of small-scale coronal structuring. Title: Damping of Torsional Modes in the Solar Corona Authors: Copil, Paula; Voitenko, Yuriy; Goossens, Marcel Bibcode: 2007AIPC..895..147C Altcode: The Alfvén wave is one of the classic waves in magnetoplasma. It is an electromagnetic-hydrodynamic wave, in which the restoring force comes from the magnetic tension, while the ions provide the inertia. We have studied the propagation and dissipation of torsional Alfvén waves in an inhomogeneous cylindrical plasma taking into account the effects due to finite Larmor gyroradius. Title: Energization of Plasma Species by Intermittent Kinetic Alfvén Waves Authors: Voitenko, Yuriy; Goossens, Marcel Bibcode: 2006SSRv..122..255V Altcode: We propose a new phase-mixing sweep model of coronal heating and solar wind acceleration based on dissipative properties of kinetic Alfvén waves (KAWs). The energy reservoir is provided by the intermittent ∼1 Hz MHD Alfvén waves excited at the coronal base by magnetic restructuring. These waves propagate upward along open magnetic field lines, phase-mix, and gradually develop short wavelengths across the magnetic field. Eventually, at 1.5-4 solar radii they are transformed into KAWs. We analyze several basic mechanisms for anisotropic energization of plasma species by KAWs and find them compatible with observations. In particular, UVCS (onboard SOHO) observations of intense cross-field ion energization at 1.5-4 solar radii can be naturally explained by non-adiabatic ion acceleration in the vicinity of demagnetizing KAW phases. The ion cyclotron motion is destroyed there by electric and magnetic fields of KAWs. Title: Magnetic interfaces in the solar atmosphere: waves, instabilities and energy release Authors: Voitenko, Y.; Siversky, T.; Copil, P.; Goossens, M. Bibcode: 2006cosp...36.3364V Altcode: 2006cosp.meet.3364V Numerous Yohkoh and SOHO observations suggest that the events of impulsive plasma heating in the solar atmosphere flares nanoflares blinkers etc are due to the energy released during magnetic reconnection Magnetic reconnection occurs in magnetic interfaces between interacting magnetic fluxes Classical transport coefficients cannot explain the observed rates of energy release As a consequence several current-driven plasma micro-instabilities have been suggested as mechanisms causing anomalous resistivity and faster energy release The common difficulty of models based on the current-driven instabilities is that the threshold currents for these instabilities are rather high and require very thin interfaces which are subject to quick disruption In this situation the fast Petschek regime of magnetic reconnection can hardly be obtained In our study we take into account that inhomogeneous shear plasma flows and currents as well as considerable guide magnetic field components are typical for coronal magnetic interfaces We find that the shear plasma flows and current inhomogeneity drastically decrease the threshold currents for kinetic Alfven and ion-acoustic instabilities As a result these instabilities can develop anomalous resistivity much earlier in relatively smooth and stable interfaces which make the standard Petschek model more realistic for the solar corona Moreover inhomogeneous currents that are typical for the quasi-steady solar corona can also drive these instabilities which can therefore contribute to the quasi-steady heating of the corona Title: Non-adiabatic acceleration of ions by kinetic Alfven waves Authors: Voitenko, Y.; Goossens, M. Bibcode: 2006cosp...36.3372V Altcode: 2006cosp.meet.3372V Strong energization of ions across the background magnetic field is one of most interesting observations in the solar corona at 1 5-4 solar radii and in the auroral zones of the terrestrial magnetosphere at 1 5-4 Earth radii The commonly accepted interpretation of this phenomenon is based on the ion-cyclotron resonant heating by high-frequency waves in the solar corona or stochastic heating by small-scale waves in the auroral zones We propose another mechanism where the cross-field ion energization is due to non-adiabatic acceleration by low-frequency kinetic Alfven waves KAWs In the vicinity of even demagnetizing wave phases all ions undergo a simultaneous increase of their cross-field velocities similar to particle acceleration in quasi-perpendicular shocks It is therefore intuitively understandable why the particles that move against the waves enter the regime of acceleration easier In-situ measurements of electro-magnetic fields in the auroral zones and remote spectroscopic coronal observations are compatible with low-frequency KAW turbulence We demonstrate that the sporadic appearance of super-critical gradients in KAW turbulence is sufficient for the cross-field energization of ions observed in these regions Title: Voitenko and Goossens Reply: Authors: Voitenko, Y.; Goossens, M. Bibcode: 2005PhRvL..95z9502V Altcode: A Reply to the Comment by P. K. Shukla and L. Stenflo. Title: Anomalous Viscous Dissipation of Slow Magneto-Acoustic Waves Authors: Siversky, T.; Voitenko, Y.; Goossens, M. Bibcode: 2005ESASP.600E..99S Altcode: 2005ESPM...11...99S; 2005dysu.confE..99S No abstract at ADS Title: Phase Mixing of MHD ALFVÉN Waves and Origin of Solar Wind Authors: Voitenko, Y.; Goossens, M. Bibcode: 2005ESASP.600E.103V Altcode: 2005ESPM...11..103V; 2005dysu.confE.103V No abstract at ADS Title: Foreword: Computing in Space and Astrophysical Plasmas Authors: Goossens, Marcel; Poedts, Stefaan; Voitenko, Yuriy; Chian, Abraham C. -L. Bibcode: 2005SSRv..121....1G Altcode: No abstract at ADS Title: Shear Flow Instabilities in Low-Beta Space Plasmas Authors: Siversky, Taras; Voitenko, Yuriy; Goossens, Marcel Bibcode: 2005SSRv..121..343S Altcode: We study instabilities driven by a sheared plasma flow in the low-frequency domain. Two unstable branches are found: the ion-sound mode and the kinetic Alfvén mode. Both instabilities are aperiodic. The ion-sound instability does not depend on the plasma β (gas/magnetic pressure ratio) and has a maximum growth rate of about 0.1 of the velocity gradient dV 0/dx. On the other hand, the kinetic Alfvén instability is stronger for larger β and dominates the ion-sound instability for β > 0.05. Possible applications for space plasmas are shortly discussed. Title: Nonlinear coupling of Alfvén waves with widely different cross-field wavelengths in space plasmas Authors: Voitenko, Yuriy M.; Goossens, Marcel Bibcode: 2005JGRA..11010S01V Altcode: Multiscale activity and dissipation of Alfvén waves play an important role in a number of space and astrophysical plasmas. A popular approach to study the evolution and damping of MHD Alfvén waves assumes a gradual evolution of the wave energy to small dissipative length scales. This can be done by local nonlinear interactions among MHD waves with comparable wavelengths resulting in turbulent cascades or by phase mixing and resonant absorption. We investigate an alternative nonlocal transport of wave energy from large MHD length scales directly into the dissipation range formed by the kinetic Alfvén waves (KAWs). KAWs have very short wavelengths across the magnetic field irrespectively of their frequency. We focus on the nonlinear mechanism for the excitation of KAWs by MHD Alfvén waves via resonant decay AW → KAW1 + KAW2. The resonant decay conditions can be satisfied in a rarified plasmas, where the gas/magnetic pressure ratio is less than the electron/ion mass ratio. The decay is efficient at low amplitudes of the magnetic field in the MHD waves, B/B0 ∼ 10-2. In turn, the nonlinearly driven KAWs have sufficiently short wavelengths for the dissipative effects to become significant. Therefore the cross-scale nonlinear coupling of Alfvén waves can provide a mechanism for the replenishment of the dissipation range and the consequent energization in space plasmas. Two relevant examples of this scenario in the solar corona and auroral zones are discussed. Title: Damping of phase-mixed slow magneto-acoustic waves: Real or apparent? Authors: Voitenko, Y.; Andries, J.; Copil, P. D.; Goossens, M. Bibcode: 2005A&A...437L..47V Altcode: The propagation of slow magnetoacoustic waves along a multithreaded coronal loop is modelled analytically by means of a ray tracing method. It is shown how cross field gradients build up due to phase mixing. The cross field gradients can enhance shear viscosity so that it dominates over compressive viscosity. Nevertheless the short dissipation distances (~107 m) observed for slow waves in coronal loops require very small cross field length scales which imply a filamentary structure on scales at least three orders of magnitude below the current detection limit of TRACE and close to the limit where magnetohydrodynamic (MHD) theory breaks down. The observed dissipation distances can alternatively be explained by phase mixing in its ideal regime, where the apparent damping is due to the spatial integration of the phase mixed amplitudes by the observation. Title: Cross-Scale Nonlinear Coupling and Plasma Energization by Alfvén Waves Authors: Voitenko, Y.; Goossens, M. Bibcode: 2005PhRvL..94m5003V Altcode: We present a new channel for the nonlocal transport of wave energy from the large (MHD) scales to the small (kinetic) scales generated by the resonant decay of MHD Alfvén waves into kinetic Alfvén waves. This process does not impose any restriction on the wave numbers or frequencies of initial MHD waves, which makes it superior compared to the mechanisms of spectral transport studied before. Because of dissipative properties of the nonlinearly driven kinetic Alfvén waves, the decay leads to plasma heating and particle acceleration, which is observed in a variety of space and astrophysical plasmas. Two examples in the solar corona and the terrestrial magnetosphere are briefly discussed. Title: Cross-Field Heating of Coronal Ions by Low-Frequency Kinetic Alfvén Waves Authors: Voitenko, Yuriy; Goossens, Marcel Bibcode: 2004ApJ...605L.149V Altcode: Low-frequency kinetic Alfvén waves (KAWs) are studied as a possible source for the strong heating of ions across the magnetic field in the solar corona. It is shown that test ions moving in the electromagnetic fields of KAWs undergo an increase in their cross-field energy because of the superadiabatic acceleration in the vicinity of the demagnetizing wave phases. In particular, it is found that KAW wave trains, with a transversal wavelength of the order of 40 proton gyroradii and with a peak wave/background magnetic field ratio >~0.1, increase the cross-field energy of O5+ oxygen ions by 1-2 orders. The required short perpendicular wavelengths can be produced by the phase mixing of MHD Alfvén waves, propagating upward from the coronal base. The superadiabatic acceleration provides an alternative to the ion-cyclotron explanation for the intense transverse heating of O+5 and Mg9+ ions observed by the Solar and Heliospheric Observatory at 1.5-3 solar radii. Title: Radio signatures of Langmuir-Alfvén turbulence in the solar atmosphere Authors: Chian, A. C. -L.; Goossens, M.; Miranda, R. A.; Rempel, E. L.; Sirenko, O.; Voitenko, Y. Bibcode: 2004IAUS..223...95C Altcode: 2005IAUS..223...95C Radio emissions from the solar active regions can be generated by nonlinear coupling of Langmuir waves with Alfvén waves. Multi-wavelength observations can be used to provide evidence for Langmuir-Alfvén turbulence in the solar atmosphere. Title: Ion Heating across the Magnetic Field in the Solar Corona by Kinetic Alfvén Waves Authors: Voitenko, Y.; Goossens, M. Bibcode: 2004ESASP.547..381V Altcode: 2004soho...13..381V The perpendicular heating of the ions observed by SOHO in the solar corona at 2-4 solar radii has been mainly attributed to the ion-cyclotron damping of high-frequency Alfvén waves. We investigate an alternative mechanism of heating by low-frequency Alfvén waves that have short wavelengths across the magnetic field - kinetic Alfvén waves (KAWs). The energy reservoir for these kinetic waves is provided by low-frequency large-scale MHD waves that are launched in the corona by the photospheric motions or excited at the coronal base by magnetic restructuring. The short perpendicular wavelengths, developed by phase mixing, convert MHD Alfvén waves into KAWs. KAWs can be also excited in situ by various linear and nonlinear mechanisms. We show that above a threshold value of the wave amplitude, KAWs can stochastically accelerate ions across the background magnetic field. In particular, KAWs with transversal wavelengths of the order of the ion inertial length and with a wave/background magnetic field ratio of the order 0.1, can contribute to the stochastic heating of oxygen ions O5+ . We discuss advantages of this mechanism over the ion-cyclotron heating scheme for the intense transverse heating of ions observed by SOHO at 2-4 solar radii. Title: Kinetic Excitation Mechanisms for ION-Cyclotron Kinetic Alfvén Waves in Sun-Earth ConnectionI Authors: Voitenko, Yuriy; Goossens, Marcel Bibcode: 2003SSRv..107..387V Altcode: We study kinetic excitation mechanisms for high-frequency dispersive Alfvén waves in the solar corona, solar wind, and Earth's magnetosphere. The ion-cyclotron and Cherenkov kinetic effects are important for these waves which we call the ion-cyclotron kinetic Alfvén waves (ICKAWs). Ion beams, anisotropic particles distributions and currents provide free energy for the excitation of ICKAWs in space plasmas. As particular examples we consider ICKAW instabilities in the coronal magnetic reconnection events, in the fast solar wind, and in the Earth's magnetopause. Energy conversion and transport initiated by ICKAW instabilities is significant for the whole dynamics of Sun-Earth connection chain, and observations of ICKAW activity could provide a diagnostic/predictive tool in the space environment research. Title: Nonlinear wave dynamics in the dissipation range Authors: Voitenko, Y.; Goossens, M. Bibcode: 2003PADEU..13..153V Altcode: There is abundant observational evidence that the ions in the solar corona (in particular, O(+5) ) are heated anisotropicaly, predominantly across the background magnetic field. This heating is usually attributed to the dissipation of ion-cyclotron waves. We study an alternative possibility with the dissipation range in the solar corona formed by the kinetic Alfvén waves (KAWs) which are very short- wavelengths across the magnetic field. Instead of transport of MHD wave energy towards to the range of ion-cyclotron waves, we study transport into the dissipation range of KAWs. We show that the nonlinear excitation of short-wavelength (of the order 10 m) KAWs in the extended solar corona and solar wind can be provided by upward-propagating fast and Alfvén MHD waves launched from the coronal base by the convection or magnetic reconnection. KAWs are very efficient in the energy exchange with plasma particles, providing plasma heating and particles acceleration. In particular, these transversal wavelengths make KAWs accessible for the stochastic perpendicular heating of oxygen ions when the wave/background magnetic field ratio exceeds 0.005. Both the quasi-steady coronal heating and the transient heating events observed by Yohkoh and SOHO may be due to KAWs that are nonlinearly excited by MHD waves. Title: Nonlinear excitation of small-scale Alfvén waves by fast waves and plasma heating in the solar atmosphere Authors: Voitenko, Yuriy; Goossens, Marcel Bibcode: 2002SoPh..209...37V Altcode: We study a nonlinear mechanism for the excitation of kinetic Alfvén waves (KAWs) by fast magneto-acoustic waves (FWs) in the solar atmosphere. Our focus is on the excitation of KAWs that have very small wavelengths in the direction perpendicular to the background magnetic field. Because of their small perpendicular length scales, these waves are very efficient in the energy exchange with plasmas and other waves. We show that the nonlinear coupling of the energy of the finite-amplitude FWs to the small-scale KAWs can be much faster than other dissipation mechanisms for fast wave, such as electron viscous damping, Landau damping, and modulational instability. The nonlinear damping of the FWs due to decay FW = KAW + KAW places a limit on the amplitude of the magnetic field in the fast waves in the solar corona and solar-wind at the level B/B0∼10−2. In turn, the nonlinearly excited small-scale KAWs undergo strong dissipation due to resistive or Landau damping and can provide coronal and solar-wind heating. The transient coronal heating observed by Yohkoh and SOHO may be produced by the kinetic Alfvén waves that are excited by parametric decay of fast waves propagating from the reconnection sites. Title: Excitation of high-frequency Alfvén waves by plasma outflows from coronal reconnection events Authors: Voitenko, Yuriy; Goossens, Marcel Bibcode: 2002SoPh..206..285V Altcode: We study a kinetic excitation mechanism for high-frequency dispersive Alfvén waves in the solar corona by magnetic reconnection events. The ion-cyclotron and Cerenkov kinetic effects are important for these waves which we call the ion-cyclotron kinetic Alfvén waves (IC KAWs). The plasma outflowing from the reconnection site sets up a neutralized proton beam in the surrounding plasma, providing free energy for the excitation of waves. The dependence of the phase velocity of the IC KAW on the parallel wavenumber is different from that on the perpendicular wavenumber. The phase velocity is an increasing function of the perpendicular wavenumber and overtakes the Alfvén velocity for sufficiently large values of k⊥. However, the phase velocity is a decreasing function of k∥, and sufficiently large values of k∥ result in a phase velocity below the Alfvén velocity. As a result, the IC KAWs can undergo the Cerenkov resonance with both super- and sub-Alfvénic particles, and for the waves to be excited the outflow velocity does not need to be super-Alfvénic, as for KAWs, but the beam/Alfvén velocity ratio can span a wide range of values. High growth rates of the order of γ∼104 s−1 are found for the values of the plasma parameters typical for the low solar corona. The waves excited by (sub-)Alfvénic beams are damped mainly due to kinetic wave-particle interactions with ions at the cyclotron resonance (ion-cyclotron damping), and with ions and electrons at the Cerenkov resonance (Landau damping). Therefore, IC KAWs can heat the plasma species of the corona in both the parallel and perpendicular direction, giving rise to an anisotropic heating of the ions. The observational consequences of the processes under study are discussed. Title: Nonlinear Damping of Fast Waves and Plasma Heating in the Solar Corona Authors: Voitenko, Y.; Goossens, M. Bibcode: 2001IAUS..203..517V Altcode: Fast waves can be excited in the corona by compressional perturbations of magnetic field lines which are anchored into the dense convective zone and displaced by the plasma motions there. The consequent linear dissipation of fast waves in the resonant layers can contribute to coronal heating. A difficulty of this dissipation mechanism is that the setup time of the linear resonance (the time required for the creation of sufficiently short length-scales) is long compared to the sub-minute variations in the coronal heating process. This suggests a more effective mechanism for the structurization of waves in the solar corona. We propose a new, nonlinear mechanism for the dissipation of fast waves in the corona. In the framework of two-fluid MHD we show that fast waves are nonlinearly coupled to the kinetic Alfvén waves - Alfvén waves with short wavelengths across B0, background magnetic field. The nonlinear coupling is effective for the amplitudes of the launched fast waves in the range 0.01 to 0.03 for B/B0 (B is wave magnetic field), implied by spectroscopic observations. As the excited AWs have very short wavelengths, they are damped almost immediatelly by the linear kinetic or collisional dissipation. Therefore, the resulting plasma heating has the overall timescale of the order of the characteristic time of nonlinear interaction, which can easily be in the sub-minute range. Title: Nonlinear Evolution of Phase-mixed Alfvén Waves Towards Short Length Scales Authors: Gossens, M.; Voitenko, Y. Bibcode: 2001IAUS..203..492G Altcode: Classic Alfvén waves (AWs) can be converted into kinetic Alfvén waves (KAWs) by phase mixing in inhomogeneous space plasmas, such as solar corona, solar wind and other astrophysical objects. Linear phase mixing on its own appears not to be able to provoke significant kinetic effects in AWs propagating initially along magnetic field. Here we present mechanisms that are more efficient once phase mixing has created the perpendicular wavenumber in a small-amplitude AW. In the framework of two-fluid MHD we show that the initial AW can nonlinearly couple its energy to other, secondary AWs, resulting in nonlinear spectral transfer of wave energy. The nonlinear generation of secondary waves can initiate a turbulent cascade of energy towards smaller scales and higher frequencies, where kinetic dissipation mechanisms of KAWs due to Landau and cyclotron damping are of a prime importance. The recent SOHO observations of the solar corona and solar wind, implying dissipation of small-scale and/or high-frequency Alfvén waves, are discussed in the light of our theoretical results. We suggest that the spectral dynamics and consequent dissipation of KAWs introduced by the combined action of phase mixing and nonlinear interaction is a widespread phenomenon, important for astrophysical plasmas. Title: Competition of damping mechanisms for the phase-mixed Alfvén waves in the solar corona Authors: Voitenko, Y.; Goossens, M. Bibcode: 2000A&A...357.1086V Altcode: The competition of the linear and nonlinear damping mechanisms for phase-mixed Alfvén waves in the solar corona is studied. It is shown that the nonlinear damping of the phase-mixed Alfvén waves due to their parametric decay is stronger than both collisional and Landau damping for waves with frequencies below a critical frequency which depends on the wave amplitude. This critical frequency is close to the cyclotron frequency ( ~ 105 s-1 in holes) even for small wave amplitudes of the order of 1% of the background value for the magnetic field. This means that the dissipation of the Alfvén wave flux in the corona can be significantly affected by the nonlinear wave dynamics. Nonlinear decay of the low-frequency Alfvén waves transmits a part of the wave energy from the length-scales created by phase mixing to smaller scales, where the waves damp more strongly. However, the direction of the effect can be reversed in the high-frequency domain, 10 s-1<allowbreak omega <allowbreak 104 s-1, where the decay into counterstreaming waves is strongest, because the wave energy is quickly transferred to larger scales, where the actual dissipation is reduced. These effects are introduced by the vector nonlinearity which involves waves propagating in the different directions across magnetic field. The effects introduced by the scalar nonlinearity may also become important in phase mixing (Voitenko & Goossens, in preparation). Title: Impulsive Flare Plasma Energization in the Light of YOHKOH Discoveries Authors: Voitenko, Y. M. Bibcode: 1998IAUS..188..211V Altcode: Spatially resolved Yohkoh and ground-based observations of solar flares strongly suggest impulsive plasma energization mechanism acting in quickly evolving transitory magnetic structures, intermediate between pre-reconnected larger-scale and post-reconnected compact SXR flare loops. For elementary just-reconnected (magnetic flux-) tube (JRT) we develop a kinetic model, based on the modern magnetic reconnection theory and Yohkoh observations. An essential features of our model are proton beams streaming down along JRT legs, and plasma density enhancements at the JRT toop and chromospheric foot-points. Dominant loss of the proton beam kinetic energy in the JRT legs is via intermediate kinetic Alfven waves (KAWs), excited by beam-driven KAW instability. The dissipation of KAW flux give rise to the fast plasma heating and enhanced flux of runaway electrons, followed by impulsive HXR emission from JRT top and footpoints, and microwaves from JRT legs. If reconnecting magnetic field is 150 G, high enough coronal temperatures ~108K may be achieved abowe SXR flare loop within timescales of ~1s. Nonlinear KAWs interaction leads to the turbulent wave energy cascade towards transversal scales kperp -1 ~rhoi, where different character of KAWs dissipation results in the ~0.1s spikes on the time profiles of impulsive plasma heating and consequent emissions. The model is successfully applied to a few concrete events.