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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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)
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]
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
M A C R O ( M H D )
Kinetic Alfvén wave (KAW) -
extension of Alfvén mode in the range of high perpendicular wavenumbers.
Padé approximation for the KAW dispersion:
- proton gyroradius.
Power Spectral Density
( f ~ k_perp )
Sahraoui et al. (2010): high-resolution magnetic spectrum
exhibits 4 different slopes (!)
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):
Non-dispersive range (MHD):
Weakly dispersive range (WDR kinetic):
Strongly dispersive range (SDR kinetic):
(Voitenko and De Keyser, 2011)
Three possible interpretations:
(1) dissipative (left), or
(2) dispersive (right), or
( f ~ k_perp )
KAW range = WDR KAW range + SDR KAW range
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.
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)
Kinetic-scale Alfvénic turbulence covers the tails’ velocity ranges
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)
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
proton diffusion occurs here
Generation of proton tails by turbulence
( f ~ k_perp ~ Vz)
WHICH IS COUNTER-INTUITIVE
Alexandrova et al. (2008)
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.