6

Waves, structures and turbulences: 

Waves, structures and turbulences Fluctuations: scales and parameters Magnetohydrodynamic waves Structures and Alfvénic fluctuations Turbulence spectra and evolution Plasma waves and dispersion Shocks and discontinuities

Slide2: 

Mikic & Linker, 1999 The Sun‘ open magnetic field lines MHD model field during Ulysses crossing of ecliptic in early 1995

Slide3: 

Length scales in the solar wind Macrostructure - fluid scales Heliocentric distance: r 150 Gm (1AU) Solar radius: Rs 696000 km (215 Rs) Alfvén waves:  30 - 100 Mm Microstructure - kinetic scales Coulomb free path: l ~ 0.1 - 10 AU Ion inertial length: VA/p (c/p) ~ 100 km Ion gyroradius: rL ~ 50 km Debye length: D ~ 10 m Helios spacecraft: d ~ 3 m Microscales vary with solar distance!

Slide4: 

Solar wind stream structure and heliospheric current sheet Alfven, 1977 Parker, 1963

Slide5: 

Spatial and temporal scales Phenomenon Frequency Period Speed (s-1) (day) (km/s) Solar rotation: 4.6 10-7 25 2 Solar wind expansion: 5 - 2 10-6 2 - 6 800 - 250 Alfvén waves: 3 10-4 1/24 50 (1AU) Ion-cyclotron waves: 1 - 0.1 1 (s) (VA) 50 Turbulent cascade: generation + transport  inertial range  kinetic range + dissipation

Slide6: 

Plasma waves and frequencies Solar distance /RS Frequency/Hz Gurnett, 1978 Non-uniformity leads to strong radial variations of the plasma parameters!

Slide7: 

Power spectrum of fluctuations Log( frequency /Hz) Mangeney et al., 1991 (a) Alfvén waves (b) Slow and fast magnetosonic (c ) Ion-cyclotron (d) Whistler mode (e) Ion acoustic, Langmuir waves

Slide8: 

Magnetohydrodynamic waves Magnetosonic waves compressible - parallel slow and fast - perpendicular fast Cms = (cs2+VA2)-1/2 Alfvén wave incompressible parallel and oblique VA = B/(4)1/2

Slide9: 

Phase velocities of MHD modes 4 - 2 (kcms)2 + (kcs)2 (k•VA)2 = 0  = k•VA

Slide10: 

Alfvénic fluctuations Neubauer et al., 1977 V =  VA Helios

Slide11: 

Alfvén waves and solar wind streams Tu et al., GRL, 17, 283, 1990 High wave flux in fast streams Developed turbulence in slow streams

Slide12: 

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

Slide13: 

Alfvénic fluctuations (Ulysses) Horbury & Tsurutani, 2001 Elsässer variables: Z = V  VA Turbulence energy: e = 1/2 (Z±)2 Cross helicity: c = (e+ - e-)/(e+ + e-)

Slide14: 

Power spectrum Horbury et al., JGR 101, 405, 1996 Turbulence spectrum: e(f) = 1/2 (Z±)2  (f/f0)-  5/3 1

Slide15: 

Spectral evolution and turbulent cascade: slope steepening

Slide16: 

Kolmogorov phenomenology for isotropic homogeneous turbulence Energy cascade: Turbulent energy (per unit mass density), el  (Z)2, at scale l is transported by a hierarchy of turbulent eddies of ever decreasing sizes to the dissipation range at scale lD. l  Z/l (Z)2  ek3/2 k5/2 energy transfer rate: l  (Zl)2/ turnover time:   l /Zl wavenumber: k  1/l energy spectrum: ekk  (Zl)2 Scale invariance: l =  (dissipation rate) -->   k-5/3

Slide17: 

Daily fluctuations of energy spectra Out: e+(f) (units of 1 km2s-2/day) In: e-(f) Grappin et al., J. Geophys. Res. 95, 8197, 1990 Self-similar fluctuations fn = 1/day 2n e(f9)  cs e(f1)  n/n

Slide18: 

Turbulence in the heliosphere Questions and problems: Nature and origin of the fluctuations Distribution and spectral transfer of turbulent energy Spatial evolution with heliocentric distance 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 >

Slide19: 

Radial evolution of spectral densities Bavassano et al., J. Geophys. Res. 87, 3617, 1982 B magnitude B trace of auto- correlation matrix

Spectral evolution of Alfvénic fluctuations: 

Spectral evolution of Alfvénic fluctuations High-frequency waves in the corona? 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

Slide21: 

Spectral indices and spatial evolution of turbulence Marsch and Tu, JGR, 95, 8211, 1990 Spectra steepen! e+  e- , Alfvén waves dominate! -5/3 slow <-> fast wind

Slide22: 

Solar wind turbulence Parameter Coronal Hole Current sheet (open) (closed) Alfvén waves: yes no Density fluctuations: weak (<3%) intense (>10%) Magnetic/kinetic  1 > 1 turbulent energy: Spectral slope: flat (-1) steep (-5/3) Wind speed: high low Tp (Te): high (low) low (high) Wave heating: strong weak

Slide23: 

Radial variation of spectral features Turbulence intensity declines with solar distance Wave amplitudes is consistent between Helios and Ulysses in fast streams from coronal holes Variation of spectral breakpoint (decreases) as measured by various spacecraft Slower radial evolution of spectra over the poles Horbury & Tsurutani, 2001

Slide24: 

Stream interaction region Dynamic processes in inter-planetary space Wave amplitude steepening (n~ r-2) Compression and rarefaction Velocity shear Nonlinearity by advection (V)V Shock formation (co-rotating)

Slide25: 

Compressive fluctuations in the solar wind Marsch and Tu, JGR, 95, 8211, 1990 Kolmogorov-type turbulence

Slide26: 

Correlation length of turbulence Helios, Voyager Lc = Vswc Lc Correlation function: CAA‘(x,t,x‘,t‘) = <A(x,t) A(x‘,t‘)> for any field A(x,t). If stationarity and homogeneity, then  = t-t‘, r = x-x‘ CAA‘(x,t,x‘,t‘) = CAA‘(r, )

Slide27: 

Integral invariants of ideal MHD E = 1/2  d3x (V2 + VA2) Energy Hc =  d3x (V • VA) Helicity Hm =  d3x (A • B) Magnetic helicity B =  x A Elsässer variables: Z± = V ± VA E± = 1/2  d3x (Z±)2 =  d3x e±(x)

Slide28: 

eA(k) = 1/2  d3k e-i k•r<A(0)•A(r)> Alfvén ratio rA(k)= eV(k)/eB(k) Spectrum:

Slide29: 

Cross helicity c(f) = ec(f)/e(f) = (e+ - e-)/(e+ + e-) Tu et al., J. Geophys. Res. 95, 1739 , 1989 High Alfvénic correlations

Slide30: 

Evolution of cross helicity c = 2<V • VA> /(V2 + VA2) = (e+ - e-)/(e+ + e-) Roberts et al., J. Geophys. Res. 92, 12023 , 1987 Alfvénic correlations

Slide31: 

MHD turbulence dissipation through absorption of dispersive kinetic waves Viscous and Ohmic dissipation in collisionless plasma (coronal holes and fast solar wind) is hardly important Waves become dispersive (at high frequencies beyond MHD) in the multi-fluid or kinetic regime Turbulence dissipation involves absorption (or emission by instability) of kinetic plasma waves! Cascading and spectral transfer of wave and turbulence energy is not well understood in the dispersive dissipation domain!

Slide32: 

The Helios spacecraft Twin spacecraft for heliospheric physics in highly eccentric orbits with perihelion at 0.3 AU during the years 1974-1986

Slide33: 

Phase velocities of wave modes Gurnett, 1978 Doppler shift: ’ = +kVsw

Slide34: 

Electrostatic waves Plasma frequency pe2 = 4e2ne/me Ion acoustic speed cia2 = ekBTe/mi Debye length D2 = kBTe/(4e2ne)

Slide35: 

Ion acoustic and Langmuir waves Gurnett, 1991

Slide36: 

Electric field power spectrum Gurnett & Anderson, JGR 82, 632, 1977 No power law but hump Abrupt decline at fp- indicates electron Landau damping (absorption) Spectrum at frequencies between 103 Hz and 5x104 Hz is mainly due to Doppler-shifted ion acoustic waves

Slide37: 

Gurnett et al., JGR 84, 541, 1979 Ion acoustic waves at a shock  = s + kV s = csk/(1+k2D2)1/2

Slide38: 

Electromagnetic waves Ion gyrofrequency gi = qiB/(mic) Upper/lower hybrid frequency uh2 = pe2 + ge2 ; lh2 = gegi

Slide39: 

Whistler mode waves at a shock Gurnett et al., JGR 84, 541, 1979 w = pe(kc/ge)2

Slide40: 

Magnetic field power spectrum Power laws with index of about -1, -5/3 and -3 Abrupt decline at fc indicates cyclotron absorption Steep spectrum at high frequencies above 2 Hz is mainly due to whistler waves Denskat et al., JGR 54, 60, 1983

Oxygen and hydrogen velocities in coronal holes: 

Oxygen and hydrogen velocities in coronal holes Cranmer et al., Ap. J., 511, 481, 1998 Preferential acceleration of oxygen! Magnetic mirror in polar coronal hole Cyclotron resonance  increase of O Outflow velocities

Slide42: 

Lin et al., in SW9, 673, 1999 Ion acoustic waves for Te/Tp>1 Solar wind data in a magnetic cloud

Slide43: 

Ulysses wave data - day 73 in 1995 McDowell and Kellog, 2001 Power in grey scale in dB above noise Electron beam driven

Slide44: 

Discontinuities and shocks Contact discontinuity (CD) Index 1 upstream and 2 downstream; B does not change across the surface of the CD, but 1  2 and T1  T2 . Continuity of the mass flux and magnetic flux: Bn = B1n = B2n Gn = 1(V1n-U) = 2(V2n-U) U is the speed of surface in normal direction; B magnetic field vector; V flow velocity. Mach number: M = V/C; C is the wave phase speed. Shock: G  0 Discontinuity: G = 0

Slide45: 

Shocks (with mass flow) Parallel shock Perpendicular shock B is perpendicular to the normal n of the shock surface. B is parallel to the normal n of the shock surface. B1/B2 = 1/2

Slide46: 

Fast and slow shocks Fast shock B and V refract away from normal Slow shock B and V refract towards the normal

Slide47: 

Possible geometries of shock normal and magnetic field cos(Bn) = B•n/B Laminar Turbulent

Slide48: 

Discontinuities (no mass flow) Tangential discontinuity (TD) B is parallel to the surface of the TD, but its direction may change across it. Rotational discontinuity (RD) B is oblique to the surface of the RD and its direction changes across it. Alfvén shock

Slide49: 

Alfvénic fluctuations Tsurutani et al., 1997 Ulysses observed many such waves (4-5 per hour) in fast wind over the poles: Arc polarized waves Phase-steepened Rotational discontinuity: V = ± VA Finite jumps in velocities over gyrokinetic scales

Slide50: 

Arc-polarized Alfvén waves Tsurutani et al., 1997 Rotational discontinuity RD lasts only 3 minutes Slowly rotating Alfvén wave lasts about 15 minutes