Yuriy Voitenko

Cross-Field Heating of Coronal Ions by Low-Frequency Kinetic Alfvén Waves

Low-frequency kinetic Alfven 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 Alfven 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.

Properties of Short-wavelength Oblique Alfvén and Slow Waves

Linear properties of kinetic Alfven 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. and plasma-to-magnetic pressure ratio beta. The KAW frequency can reach and exceed the ion-cyclotron frequency at ion kinetic scales, whereas the KSW frequency remains sub-cyclotron. At beta similar to 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 beta, 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 sigma similar to 1 for KAWs and sigma similar to -1 for KSWs, and the ion Alfven ratio R-Ai > 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.

Turbulent spectra and spectral kinks in the transition range from MHD to kinetic Alfvén turbulence

Abstract. A weakly dispersive range (WDR) of kinetic Alfven turbulence is identified and investigated for the first time in the context of the MHD/kinetic turbulence transition. We find perpendicular wavenumber spectra p kb−3 and p kb−4 formed in WDR by strong and weak turbulence of kinetic Alfven 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.

Nonlinear excitation of small-scale Alfvén waves by fast waves and plasma heating in the solar atmosphere

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.

Kinetic Excitation Mechanisms for Ion-Cyclotron Kinetic Alfvév Waves in Sun-Earth Connection

We study kinetic excitation mechanisms for high-frequency dispersive Alfven 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 Alfven 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.

SCALAR AND VECTOR NONLINEAR DECAYS OF LOW-FREQUENCY ALFVÉN WAVES

We found several efficient nonlinear decays for Alfven 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 Alfven wave into co- and counter-propagating product Alfven and slow waves are controlled by the scalar nonlinearities at long wavelengths (k 0⊥ is wavenumber perpendicular to the background magnetic field, ω0 is frequency of the pump Alfven 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 Alfven 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, , 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 Alfven 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.

OBLIQUE ALFVÉN INSTABILITIES DRIVEN BY COMPENSATED CURRENTS

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 Alfvenic instability driven by compensated currents flowing along the background magnetic field. Because of the vastly different electron and ion gyroradii, oblique Alfvenic 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 Earths 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.

Velocity-Space Proton Diffusion in the Solar Wind Turbulence

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.

Kinetic Alfvén turbulence below and above ion cyclotron frequency

Alfvenic 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. Key Points Dispersion and intermittent effects are included in an Alfvenic turbulence model Spectra of fully intermittent turbulence fill a narrower range of spectral indices [-7/3, -3] The kinetic Alfven wave can reach the ion cyclotron frequency through the turbulent cascade.

Torsional Alfvén waves in small scale current threads of the solar corona

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 Alfven 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 Alfven waves. Results. We show that the trapped Alfven 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 Alfven speed in the interior and the exterior Alfen speed. Conclusions. Torsional Alfven waves can be guided by thin twisted magnetic flux-tubes (current threads) in the solar corona. We suggest that the current threads guiding torsional Alfven waves, are subject to enhanced plasma heating due to wave dissipation.