turbulence in the solar wind – generation, evolution and ... · turbulence in the solar wind –...
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Turbulence in the Solar Wind – Generation, Evolution and
Dissipation
Eckart Marsch
MPI für Sonnensystemforschung, Katlenburg-Lindau
Talk in the Solar Physics Seminar on Thursday, 15 May 2008
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Water jetMHD simulation: current density
Turbulent wake behind obstacle
Turbulence
Helios 2
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• Solar corona and origin of the solar wind• Solar wind and heliospheric magnetic field• Fluctuations - scales and parameters• Magnetoacoustic and Alfvénic fluctuations• Alfvén waves in the solar corona• Turbulence spectra and radial evolution• Cross-helicity, anisotropy, compressibility• Spectral properties and cascading• Scaling behaviour and intermittency• Kinetic features and dissipation of turbulence
Overview
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Corona of the active sun
EIT - LASCO C1/C2, 1998
Schwenn and Srivastava, 2000
Fe XII 19.5 nm
Fe XIV 530.3 nm
Visible light Thomson scattering
T=25 d
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Changing corona and solar wind
McComas et al., GRL, 2000
North Heliolatitude / degree South
SOHO and Ulysses
45 30 15 0 -15 -30
-45
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Coronal magnetic field
Banaszkiewicz et al., 1998;
Schwenn et al., 1997
LASCO C1/C2 images (SOHO)
Current sheet is a symmetric disc anchored at high latitudes !
Dipolar, quadrupolar, current sheet contributions
Polar field: B = 12 G
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Heliospheric current sheet
Alfvén, 1977
Parker, 1963
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Solar wind fast and slow streams
Marsch, 1991
Helios 1976
Alfvén waves and small-scale structures
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Dynamic stream interactions
Dynamic processes in inter-planetary space
• Amplitude steepening (n ~ r-2, Br ~ r-2)
• Compression and rarefaction
• Velocity shear
• Nonlinearity by advection (V)V
• Shock formation (co-rotating)
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Temporal scales
Phenomenon Frequency Period Speed (s-1) (day) (km/s)
Solar rotation: 4.6 10-7 25 2
Solar wind expansion: 6 - 3 10-6 2 - 4 800 - 300
Alfvén waves: 3 10-4 1/24 50 (1AU) Ion-cyclotron waves: 1 - 0.1 1 (s) (VA) 50
Coulomb collisions: 10-5 - 10-6 1 - 10Turbulent cascade: generation + transport
inertial range kinetic range + dissipation
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Fluctuations
Typical day in April 1995 of Ulysses plasma and field observations in the polar (420 north) heliosphere at 1.4 AU
Horbury & Tsurutani, 2001
• Sharp changes in field direction
• Large Component variations
• Weak compressive fluctuations
radial
tangential
normal
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Incompressible MHD equations
Momentum
Induction
Actual Alfvén velocityKinematic viscosity
Magnetic diffusivity
Magnetic Reynolds number 1010 in solar wind
Background Alfvén velocity
Ptot is the total pressure (thermal plus magnetic)
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Rapid dynamic alignment
Matthaeus et al., PRL, 2008
Two-dimensional incompressible MHD simulation, showing areas where cos < -0.7 (black), I cos I < 0.7 (gray), and cos > 0.7 (white).
Alignment of flow and magnetic field vectors occurs within the turnover time t = l/v of an eddy of scale size l.
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Elsässer variables
Alfvén waves
Dissipation Forcing
Advection, nonlinearity
parallel antiparallel
z = v b
Elsässer, 1950
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Alfvénic fluctuations (Helios)
Neubauer et al., 1977 V = - VA
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Alfvénic fluctuations (Ulysses)
Horbury and Tsurutani, 2001 V = + VA
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Radial variation
Bavassano et al., JGR, 105, 15959, 2001
Elsässer ratio: re = e-/e+ based on average values over 0.1 AU wide intervals of hourly variances of z .
Radial variation of energy e(r) = 1/2 (z±)2 , with wave amplitude at 1-hour period.
WKB ~ r -1
Heliocentric distance
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Latitudinal variation
Forsyth et al., 19961-hour scale
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Magnetohydrodynamic waves
• Magnetosonic waves
compressible
- parallel slow and fast
- perpendicular fast
Cms = (Cs2+CA
2)1/2
• Alfvén wave
incompressible
parallel and oblique
CA = B/(4)1/2
Broad band in k and random phases
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Two-component model
• Alfvén waves parallel to the mean field
• 2-dimensional turbulence perpendicular
• Convected structures (discontinuities) and shocks
pressure balanced
Flux tube angular scale: 2° - 4°; supergranule: 20-30 Mm
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Compressive fluctuations
Bavassano et al., Ann. Geophysicae, 2004
Colour coding:
Positive correlations between total plasma pressure pt and density n, and kinetic (thermal) pk and magnetic pm pressure, indicating magnetoacoustic fluctuations of the slow mode type.
Left scale:
Time, radial distance, and heliographic latitude of Ulysses.
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Compressive fluctuation spectra
Marsch and Tu, JGR, 95, 8211, 1990
Kolmogorov turbulence of passive scalars
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Alfvèn wave spectra
Tu et al., GRL, 1990
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Coronal Alfvén waves
Tomczyk et al. Science, 2007
The FeXIII 1074.7nm coronal emission line seen with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico
Travel time analysis
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DePontieu et al. Science, 2007
1
2
Hinode SOT CaII H 396.8 nm
Spicular transverse motion Numerical simulation
Chromospheric Alfvén waves
FW = 100 W/m2
T = 150 – 350 s
v = 20 km/s VA = 200 km/s
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Flows and waves in prominences
Okamoto et al., Science, 2007
T = 20000 K
Ca II 396.8 nm
Hinode
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Turbulence in the heliosphere
• Nature and origin of the fluctuations
• Spatial evolution with heliocentric distance
• Distribution and spectral transfer of turbulent energy
• Intermittency and microphysics of dissipation
Scaling, non-linear couplings and cascading
Alfvénic correlations: Alfvénicity (cross helicity)
c = (e+ - e-)/(e+ + e-) = 2< V•VA>/< (V)2 + (VA)2 >
Magnetic versus kinetic energy: Alfvén ratio
rA = eV/eB = < (V)2 >/< (VA)2 >
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Evolution of cross helicity
c = 2<V • VA> /(V2 + VA
2)
= (e+ - e-)/(e+ + e-)
Roberts et al., J. Geophys. Res. 92, 12023 , 1987
Alfvénic correlations decay radially!
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eA(k) = 1/2 d3k e-i k•r<A(0)•A(r)>
Alfvén ratio
rA(k)= eV(k)/eB(k)
Marsch and Tu, J. Geophys. Res., 95, 8211, 1990
slow fast
Alfvén ratio increases radially!
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Anisotropy and correlation
„Maltese cross“
Correlations:Alfvén waves and 2-D turbulence
Matthaeus et al., J. Geophys. Res., 95, 20673, 1990
Autocorrelation function
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Radial variation of turbulence
• Turbulence intensity declines with solar distance
• Wave amplitudes are consistent between Helios and Ulysses in fast streams from coronal holes
• Variation of spectral breakpoint (decreases) as measured by various S/C
• Slower radial evolution of spectra over the poles
Horbury & Tsurutani, 2001
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Length scales in the solar wind
Macrostructure - fluid scales
• Heliocentric distance: r 150 Gm (1AU = 214 Rs)
• Solar radius: Rs 696000 km
• Alfvén waves: 30 - 100 Mm
Microstructure - kinetic scales
• Coulomb free path: lC ~ 0.1 - 10 AU
• Ion inertial length: VA/p (c/p) ~ 100 km
• Ion gyroradius: rL ~ 50 km
• Debye length: D ~ 10 m
• Spacecraft size: d ~ 3 m Microscales vary with radial distance
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Cascading and spectral transfer
Energy spectrum change = transfer – dissipation + forcing
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Kolmogorov phenomenology for isotropic homogeneous turbulence
Energy cascade: Turbulent energy (per unit mass density), el (Vl)
2,
at scale l is transported by a hierarchy of turbulent eddies of ever decreasing sizes to the dissipation range at scale lD.
l Vl/l (Vl)2 Ek
3/2 k5/2
energy transfer rate: l (Vl)2/
turnover time: l/Vl
wavenumber: k 1/l
energy spectrum: Ekk (Vl)2
Scale invariance: l = (dissipation rate) --> Ek 2/3 k-5/3
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Simulation of 3-D MHD turbulence
Ek 2/3 k-5/3 Kolmogorov, 1941
Ek (vA)1/2 k-3/2 Kraichnan, 1965
Müller and Biskamp, Phys. Rev. Lett., 84, 475, 2000
Direct numerical simulation with a spectral code with 5123 modes
Compensated normalized spectrum shows Kolmogorov scaling and sheet-like dissipative structures
-5/3
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Power spectrum evolution
Horbury et al., JGR 101, 405, 1996
Turbulence spectrum:
eB(f) = 1/2 (B)2 (f/f0)-
5/3
1
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Evidence for dissipation
• Steepening by cascading
• Ion heating by wave sweeping
• Dissipation by wave absorption
Tu and Marsch, J. Geophys. Res. , 100, 12323 ,1995
0.29 AU
0.87 AU
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• Self-similarity: Turbulent eddies look alike on any scale in the inertial and dissipation domain -> scale-invariance.
• In the absence of intrinsic scale, the fluctuations exhibit power-law behaviour with exponent h:
• However, MHD simulations and solar wind observations show that smaller eddies become increasingly sparse -> intermittency.
• Spectral transfer (inertial range) rate and dissipation rate become scale dependent and vary rapidly in space and time.
Intermittency
Fluctuation/eddy amplitude:
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Energy dissipation rate becomes scale dependent and intermittent (fractal or not space-filling). Structure function has scaling exponent p, depending on geometry.
Scaling of velocity fluctuations
Dissipative structures: 2-D current and vorticity sheets.
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Structure function scaling
Burlaga, JGR, 96, 5847, 1991
Sp()=<|V()-V(0)|p>=s(p)
Scaling exponent s(p) of speed increments
s(p) = 1- ln[Pp/3 + (1-P)p/3] P-model of fractal cascade; P=1/2 no intermittency
Voyager 2 near 8.5 AU s=p/3
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Variation of intermittency
Bruno et al., J. Geophys. Res., 108, 1130, 2003
Helios, fast solar wind: Bx radial component of magnetic field, By, Bz.
Flatness (Gaussian, 3):
fast
slow
Time scale
Slow wind more intermittent!
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Probability distribution functions
Probability distributions
Spatial intermittency
Marsch and Tu, Annales Geophys., 12, 1127, 1994
Helios: fast SW, Vx radial component of flow velocity
exp(-x')
exp(-x'2)
• Non-Gaussian statistics at small scales
• Increased probability of large fluctuations
22.5 h 13.5 min
2.25 h 81 s
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Kinetic processes in solar wind
Problems:
• Transport properties of a plasma involving multiple scales......?
• Thermodynamics of solar wind plasma far from equilibrium.....?
• Plasma is multi-component and non-uniform
Complexity beyond MHD paradigm
Kinetic physics required for dissipation
• Plasma is dilute and micro-turbulent
Free energy for micro-instabilities
Resonant wave-particle interactions
Weak and varying collisions
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Kinetic dissipation by plasma waves
Marsch, Liv. Rev. Solar Phys. 2004
Ion acoustic wave
Proton beam
Magneto-sonic wave
Ion differential streaming
Ion cyclotron wave
Proton anisotropy
Kinetic wave mode
Free energy source
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Parametric decay of Alfvén wave
• Linear Vlasov theory and 1-D hybrid simulations to study the parametric instabilities of a circularly polarized parallel-propagating Alfvén wave.
• Linear and weakly nonlinear instabilities of an Alfvén wave drive ion acoustic-like and cyclotron waves, leading to beam and anisotropic core.
Araneda, Marsch, and Viñas, JGR, 2007; and PRL 2008
Pitch-angle scattering and energy diffusion in micro-turbulent wave field
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Summary• Solar wind is a weakly anisotropic turbulent magnetofluid• Alfvénic fluctuations dominate, with an admixture of weak
compressive (slow magnetoacoustic) fluctuations• Alfvén waves mostly come from the corona, compressive
waves from pressure imbalances and stream interactions • Alfvén ratio, cross-helicity, anisotropy evolve radially, as
do the average spectra and their slopes• Turbulence develops towards Kolmogorov spectra, but
intermittency prevails at small scales• Structure functions and probability distributions clearly
reveal non-gaussian statistics• Intermittency is partly due to small convected structures,
such as rotational and tangential discontinuities• Dissipation takes place in the dispersion domain via
damping of plasma waves (ion-cyclotron, ion-acoustic)