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Alfv é nic turbulence at ion kinetic scales Yuriy Voitenko Solar-Terrestrial Centre of Excellence, BIRA-IASB, Brussels, Belgium Recent results obtained in collaboration with J. De Keyser, V. Pierrard, J. S. Zhao, D. J. Wu STORM annual meeting (25-26 November 2013, Graz, Austria). OUTLINE.

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slide1
Alfvénic turbulence at

ion kinetic scalesYuriy VoitenkoSolar-Terrestrial Centre of Excellence, BIRA-IASB, Brussels, Belgium

Recent results obtained in collaboration with

J. De Keyser, V. Pierrard, J. S. Zhao, D. J. Wu

STORM annual meeting

(25-26 November 2013, Graz, Austria)

slide2
OUTLINE

1. MHD Alfvénic turbulence evolves anisotropically toward large wavenumbers (perpendicular to the mean magnetic field)

[Goldreich and Sridhar,1996]

2. Alfvén waves at ion (proton) kinetic scales (KAWs with finite differ drastically from MHD Alfvén waves [Hasegawa and Chen, 1974]

3. Alfvén turbulence at ion kinetic scales is much less known

[see however Voitenko, 1998; Voitenko and De Keyser, 2011]

slide3
k

||

ICW

_

-

1

d

i

I o n – c y c l o t r o n

m i c r o ( k i n e t i c )

C h e r e n k o v

N o n – a d I a b a t I c

KAW

M A C R O ( M H D )

|

|

-

1

k

r

R

-

1

ç

^

i

slide4
KINETIC ALFVEN WAVES - KAWs

Kinetic Alfvén wave (KAW) -

extension of Alfvén mode in the range of high perpendicular wavenumbers.

Padé approximation for the KAW dispersion:

- proton gyroradius.

slide5
THEORY (Howes; Schekochihin et al., 2008-2013)

-5/3

Power Spectral Density

-7/3

MHD RANGE

KAW RANGE

=1

slide6
SOLAR WIND TURBULENCE

?

MHD RANGE

KAW RANGE

( f ~ k_perp )

Sahraoui et al. (2010): high-resolution magnetic spectrum

exhibits 4 different slopes (!)

slide7
MHD VS KINETIC ALFVÉN TURBULENCE

AT MHD SCALES (MHD AWs):

Only counter-propagating MHD AWs interact (Goldreich and Sridhar, 1995)

AT KINETIC SCALES (KAWs):

Counter-propagating KAWs interact (Voitenko, 1998):

Co-propagating KAWs interact (Voitenko, 1998):

slide8
ALFVÉNIC TURBULENCE SPECTRA (THEORY)

Non-dispersive range (MHD):

 weak turbulence;

 strong turbulence;

Weakly dispersive range (WDR kinetic):

 weak turbulence;

STEEPEST!

 strong turbulence;

Strongly dispersive range (SDR kinetic):

 weak turbulence;

 strong turbulence;

slide9
DOUBLE-KINK SPECTRAL PATTERN

(Voitenko and De Keyser, 2011)

Three possible interpretations:

(1) dissipative (left), or

(2) dispersive (right), or

(3) =(1)+(3)

slide10
ALFVÉNIC TURBULENCE IN SOLAR WIND

WDR

kinetic

MHD RANGE

SDR KINETIC

( f ~ k_perp )

KAW range = WDR KAW range + SDR KAW range

slide12
RECENT OBSERVATIONS OF WAVE MODES

Wave-vector inclination (top) and frequency (bottom) versus wavenumber

[Narita et al., 2011].

The dispersion analysis suggests whistlers/magnetosonic waves rather than kinetic Alfven waves.

slide13
RECENT OBSERVATIONS OF WAVE MODES

Exploiting BIIBo component to discriminate KAWs vs. FW/whistlers:

He et al. (2012):DO KINETIC ALFVEN/ION-CYCLOTRON OR FAST-MODE/WHISTLER WAVES DOMINATE?

Salem et al. (2012) :IDENTIFICATION OF KINETIC ALFVEN TURBULENCE IN THE SOLAR WIND

FW/whistlers are not supported by these observations:

He et al. (2012)

Salem et al. (2012)

slide15
PROTON VELOCITY DISTRIBUTIONS WITH SUPRATHRMAL TAILS AND ANISOTROPIC CORES IN THE SOLAR WIND (after E. Marsch, 2006)

Kinetic-scale Alfvénic turbulence covers the tails’ velocity ranges

slide16
Use kinetic Fokker-Planck equation for protons with diffusion terms due to KAWs

Calculate proton diffusion (plateo formation) time

Use observed turbulence levels and spectra

Estimate generated tails in the proton VDFs and compare with observed ones

VELOCITY-SPACE DIFFUSION OF PROTONS:

ANALYTICAL THEORY (Voitenko and Pierrard, 2013)

NUMERICAL SIMULATIONS Pierrard and Voitenko, 2013)

slide18
VELOCITY-SPACE DIFFUSION OF PROTONS: KINETIC SIMULATIONS (Pierrard and Voitenko, 2013)

Proton velocity distributions with

tails are reproduced not far from the boundary

KAW velocities cover this range

Proton VDF obtained at 17 Rs assuming a displaced Maxwellian as boundary condition at14 Rs by the Fokker-Planck evolution equation including Coulomb collisions and KAW turbulence

slide19
Fp

VTp

Vph2

Vz

proton diffusion occurs here

Generation of proton tails by turbulence

MHD RANGE

WDR

kinetic

SDR kinetic

( f ~ k_perp ~ Vz)

DIFFUSION

VA

slide21
FLATNESS DECREASES AT ION SCALES,

WHICH IS COUNTER-INTUITIVE

Alexandrova et al. (2008)

slide22
SPECTRALLY LOCALIZED SELECTIVE DISSIPATION

acceleration

AW turbulent spectrum (solid line) and ”threshold” spectrum for non-adiabatic ion acceleration (dashed line).Cross-field non-adiabatic ion acceleration is associated with the first spectral kink, where the turbulent spectral power raises above the threshold spectrum.

slide23
SUMMARY
  • Nonlinear kinetics becomes important at scales that are still larger than the ion gyroradius
  • Alfvenic turbulence is formed by weakly/mildly dispersive KAWs
  • Main dissipation mechanisms: nonlinear Landau damping and non-adiabatic ion acceleration
  • This explains many phenomena revealed by the field and particle observations:
  • Steepest spectra at ion scales and double-kink spectral pattern
  • Suprathermal ion tails and beams along MF
  • Proton heating across MF
  • spectrally localized selective dissipation removing highest amplitudes in the vicinity of the spectral break
  •  Reduced intermittency observed by Alexandrova et al. (2008)
  •  switch to weak turbulence and steepest spectra (were observed by Smith et al. 2006 and Sahraoui et al. 2010)
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