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Author name code: voitenko
ADS astronomy entries on 2022-09-14
author:"Voitenko, Yuriy M."
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Title: Initiation of Alfvénic turbulence by Alfvén wave collisions:
A numerical study
Authors: Shestov, S. V.; Voitenko, Y. M.; Zhukov, A. N.
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.
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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,
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.
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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
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 (ΔV<SUB>e</SUB>) 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<SUB>∥</SUB> ~
ω/k<SUB>∥</SUB>, where ω and k<SUB>∥</SUB> 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 ΔV<SUB>e</SUB> at the heliocentric distance
nearly larger than 10 times of the solar radius.
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Title: Model of imbalanced kinetic Alfvén turbulence with energy
exchange between dominant and subdominant components
Authors: Gogoberidze, G.; Voitenko, Y. M.
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.
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Title: Spectrum of imbalanced Alfvénic turbulence at ion-kinetic
scales in the solar wind
Authors: Gogoberidze, G.; Voitenko, Y. M.
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.
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Title: Temperature spectra in the solar wind turbulence
Authors: Gogoberidze, G.; Voitenko, Y. M.; Machabeli, G.
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.
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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.
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. <BR /> Aims: We study
a non-resonant reactive instability of Alfvén waves propagating
quasi-parallel to the background magnetic field B<SUB>0</SUB> in such
systems. <BR /> Methods: The instability is investigated analytically
in the framework of kinetic theory applied to the hydrogen plasmas
penetrated by hot proton beams. <BR /> Results: The instability arises
at parallel wavenumbers k<SUB>z</SUB> that are sufficiently large to
demagnetize the beam ions, k<SUB>z</SUB>V<SUB>Tb</SUB>/ω<SUB>Bi</SUB>
≳ 1 (here V<SUB>Tb</SUB> is the beam thermal speed along B<SUB>0</SUB>
and ω<SUB>Bi</SUB> 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 α<SUB>b</SUB> triggering the instability at
α<SUB>b</SUB> > α<SUB>b</SUB><SUP>thr</SUP>, where the threshold
value varies in a narrow range 2.43 ≤ α<SUB>b</SUB><SUP>thr</SUP>
≤ 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.
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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.
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<SUP>-4</SUP> ≲ ρ <SUB> i </SUB> k <SUB>0⊥</SUB> ≲ 1, where
the decay is essentially 3D and nonlocal, generating product Alfvén and
slow waves around the ion gyroscale. Here ρ <SUB> i </SUB> is the ion
gyroradius, and k <SUB>0⊥</SUB> is the pump Alfvén wavenumber. It
appears that, except for the smallest wavenumbers at and below {ρ
}<SUB>i</SUB>{k}<SUB>0\perp </SUB>∼ {10}<SUP>-4</SUP> 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.
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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.
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 m<SUB>e</SUB>/m<SUB>i</SUB>. 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
10<SUP>10</SUP>K for continuum emission, and can exceed 10<SUP>11</SUP>K
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.
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Title: MHD-Kinetic Transition in Imbalanced Alfvénic Turbulence
Authors: Voitenko, Yuriy; De Keyser, Johan
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}}}<SUB>0</SUB> 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}}}<SUB>0</SUB>. 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.
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Title: Imbalanced magnetohydrodynamic turbulence modified by velocity
shear in the solar wind
Authors: Gogoberidze, G.; Voitenko, Y. M.
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.
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Title: Imbalanced magnetohydrodynamic turbulence modified by velocity
shear in the solar wind
Authors: Gogoberidze, Grigol; Voitenko, Yuriy
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.
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Title: Kinetic Alfvén turbulence below and above ion cyclotron
frequency
Authors: Zhao, J. S.; Voitenko, Y. M.; Wu, D. J.; Yu, M. Y.
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.
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Title: Compensated-current instability of kinetic Alfvén waves
Authors: Malovichko, P.; Voitenko, Y.; De Keyser, J.
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.
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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.
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}<SUP>4</SUP> {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.
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Title: Generation of Proton Beams by Non-uniform Solar Wind Turbulence
Authors: Voitenko, Y.; Pierrard, V.
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.
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Title: Scalar and Vector Nonlinear Decays of Low-frequency Alfvén
Waves
Authors: Zhao, J. S.; Voitenko, Y.; De Keyser, J.; Wu, D. J.
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 <SUB>i</SUB><SUP>2</SUP>k<SUB>0\perp </SUB><SUP>2</SUP><ω
<SUB>0</SUB>/ω <SUB>ci</SUB> (k <SUB>0</SUB> is wavenumber
perpendicular to the background magnetic field, ω<SUB>0</SUB> is
frequency of the pump Alfvén wave, ρ<SUB> i </SUB> is ion gyroradius,
and ω<SUB> ci </SUB> 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 <SUB>i</SUB><SUP>2</SUP>k<SUB>0\perp
</SUB><SUP>2</SUP>\gtω <SUB>0</SUB>/ω <SUB>ci</SUB>, three-dimensional
vector decays dominate generating out-of-plane product waves. The
two-mode decays dominate from MHD up to ion scales ρ<SUB> i </SUB>
k <SUB>0</SUB> ~= 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 >0.1ω<SUB>0</SUB>
and can explain the origin of slow waves observed at kinetic scales.
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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.
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.
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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.
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 R<SUB>Ai</SUB> Lt 1 for KAWs and R<SUB>Ai</SUB> 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.
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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.
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.
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Title: Electrostatic plasma instabilities driven by neutral gas
flows in the solar chromosphere
Authors: Gogoberidze, G.; Voitenko, Y.; Poedts, S.; De Keyser, J.
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 10-10<SUP>2</SUP> cm in the lower chromosphere and
10<SUP>2</SUP>-10<SUP>3</SUP> 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.
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Title: Oblique Alfvén Instabilities Driven by Compensated Currents
Authors: Malovichko, P.; Voitenko, Y.; De Keyser, J.
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.
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) V<SUB>A</SUB>,
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,
V<SUB>Tp</SUB>/V<SUB>A</SUB>≈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.
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.
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<SUB>\bot</SUB><SUP>-3</SUP> and ∝
k<SUB>\bot</SUB><SUP>-4</SUP> 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
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.
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. <BR /> 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. <BR /> Methods: We
used the Hall MHD equations that we linearized in order to derive and
solve the eigenmode equation for the torsional Alfvén waves. <BR />
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. <BR
/> 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
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.
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.
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
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
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.
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.
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.
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.
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.
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.
2005SSRv..121....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Shear Flow Instabilities in Low-Beta Space Plasmas
Authors: Siversky, Taras; Voitenko, Yuriy; Goossens, Marcel
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 <SUB>0</SUB>/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
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 → KAW<SUB>1</SUB> + KAW<SUB>2</SUB>. 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/B<SUB>0</SUB> ∼ 10<SUP>-2</SUP>. 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.
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 (~10<SUP>7</SUP> 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.
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
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 O<SUP>5+</SUP> 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<SUP>+5</SUP> and Mg<SUP>9+</SUP> 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.
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.
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
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.
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
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/B<SUB>0</SUB>∼10<SUP>−2</SUP>. 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
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<SUB>⊥</SUB>. However,
the phase velocity is a decreasing function of k<SUB>∥</SUB>, and
sufficiently large values of k<SUB>∥</SUB> 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∼10<SUP>4</SUP> s<SUP>−1</SUP> 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.
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Title: Nonlinear Damping of Fast Waves and Plasma Heating in the
Solar Corona
Authors: Voitenko, Y.; Goossens, M.
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 B<SUB>0</SUB>,
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/B<SUB>0</SUB> (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.
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Title: Nonlinear Evolution of Phase-mixed Alfvén Waves Towards
Short Length Scales
Authors: Gossens, M.; Voitenko, Y.
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.
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Title: Competition of damping mechanisms for the phase-mixed Alfvén
waves in the solar corona
Authors: Voitenko, Y.; Goossens, M.
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 ( ~ 10<SUP>5</SUP> s<SUP>-1</SUP> 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<SUP>-1</SUP><allowbreak omega
<allowbreak 10<SUP>4</SUP> s<SUP>-1</SUP>, 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.
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 ~10<SUP>8</SUP>K may be achieved abowe
SXR flare loop within timescales of ~1s. Nonlinear KAWs interaction
leads to the turbulent wave energy cascade towards transversal scales
k<SUB>perp</SUB> <SUP>-1</SUP> ~rho<SUB>i</SUB>, 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.
