cosmic ray current-driven turbulence and mean-field dynamo effect
DESCRIPTION
Cosmic ray current-driven turbulence and mean-field dynamo effect. I. Rogachevskii, N. Kleeorin, A. Brandenburg , & D. Eichler (ApJ, submitted). MHD plasma with CRs. To be solved with induction equation and continuity equation, isothermal EOS. Introduces pseudoscalar. - PowerPoint PPT PresentationTRANSCRIPT
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Cosmic ray current-driven turbulence Cosmic ray current-driven turbulence and mean-field dynamo effectand mean-field dynamo effect
I. Rogachevskii, N. Kleeorin, A. Brandenburg, & D. Eichler(ApJ, submitted)
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MHD plasma with CRsMHD plasma with CRs
FEBJU
eic nnePt
1
ecri nnn cr4 JJB c
FBUBJBU
crcr1
41 enP
t c
To be solved with induction equationand continuity equation, isothermal EOS
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Introduces pseudoscalarIntroduces pseudoscalar
effect 0cr BJstarsin effect gΩ
/JbuBUB
t
... Bbu E
... jijjiji JB E
effect important for large-scale field in the Sun
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Comparison: alpha effect & inverse cascade in MHD (no CRs)
decaying turbulence forced turbulence
Here: magnetic helicity of opposite signs at small and large scale!
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Bi-helical fields from UlyssesBi-helical fields from Ulysses
• Taylor hypothesis– Broad k bins– Southern latitude with
opposite sign
• Small/large distances– Positive H at large k– Break point with
distance to larger k
Brandenburg, Subramanian, Balogh, & Goldstein (2011, ApJ 734, 9)
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Bell instabilityBell instability22cr42AzcB vkk
B
J
0cr
4 / kBJcJ
Bell (2004): J=2 Zirakashvili et al (2008): J=16
Continued growth in both cases! effect important?
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New simulationsNew simulations
• 5123 resolution, non-ideal (Re=Lu < 300)
• larger J parameter (80 and 800)
• most unstable k /k1= 40 and 400 (unresolved)
• measure alpha and turbulent diff. tensor
• Related to earlier work by Bykov et al. (2011)
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Bell instability Bell instability turbulence ( turbulence (JJ=80)=80)
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AnimationAnimation
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Power Power spectraspectra
• Bell mode ~25k1
• k4 spectrum
• k-5/3 at late time turbulence
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Spectral growthSpectral growth
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3 stages3 stages
• Bell instability,small scale, k/k1=40
• Accelerated large-scale growth
• Slow growth after initial saturation
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IsotropizationIsotropization
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Growth of length scaleGrowth of length scale
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Alpha tensor (test-field method)Alpha tensor (test-field method)
parallel
perp
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Normalized alpha effectNormalized alpha effect
crcr / 4/ Jc
Scales with kinetic helicity, prefactor ~0.5
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Dynamo number, turb diffDynamo number, turb diff
Critical value 1, turb diff >> microscopic value
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JJ and resolution dependence and resolution dependence
Larger resolution runs longer, can scale to higher J
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Equipartition ratio: Equipartition ratio: JJ dependent dependent
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Instantaneous Instantaneous growth rategrowth rate
• Agrees with non-ideal theory
• Amplification by many orders of magnitude
• Box Reynolds number ~ 1000
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Convergence of alphaConvergence of alpha
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Conclusions
• Growth by many orders of magnitude
• effect and turbulent diffusivity tensors
• dynamo number > 1 at late times
• k-5/3 spectrum at late times
• growth at larger length scales
• numerically converged
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Inverse cascadeInverse cascade
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Animation (poor)Animation (poor)
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Animation5Animation5
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Inverse cascade of magnetic helicityInverse cascade of magnetic helicity
kqp EEE |||||| kqp HHH and
||2 pp HpE ||2 qq HqE Initial components fully helical: and
||||||2|||| qpkkqp HHkHkEHqHp
),max(||||
||||qp
HH
HqHpk
qp
qp
argument due to Frisch et al. (1975)
k is forcedto the left