Excitation and Multi-scale development of Kelvin-Helmholtz (KH) waves at the Earth’s magnetopause
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Excitation and Multi-scale development of Kelvin-Helmholtz (KH) waves
at the Earth’s magnetopause
H. Hasegawa(1), A. Retinò(2), A. Vaivads(3), Y. Khotyaintsev(3), M. André(3), T. K. M.
Nakamura(1), S. Eriksson(4), W.-L. Teh(4), B. U. Ö. Sonnerup(5), S. J. Schwartz(6), & H. Rème(7)
(1) ISAS/JAXA, (2) Austrian Academy of Sci., (3) Swedish Inst. Space Phys., (4) LASP, Univ. of Colorado, (5) Dartmouth
Coll., (6) Imperial Coll. London, (7) CESR
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Shocked solar wind
Magnetopause KH instability
Nakamura et al., 2004;Hasegawa et al., 2004
KH vortices may play a key role in transport of solar wind plasma into the Earth’s magnetosphere, namely, anomalous transport of collision-less plasma.
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How does the KHI evolve?
• Worth to answer from the viewpoint of both “turbulence” and “anomalous transport of plasma”.
• How does KH wave energy cascade?
• How does the evolution of MHD-scale KH vortices set the condition for the onset of non-MHD (transport) processes?
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Cluster event on 20 Nov 2001• Extended northward
IMF• Cluster @19 MLT
~3 Re behind terminator• Rolled-up KH vortices
identified(Hasegawa et al., 2004, 2006;
Chaston et al., 2007; Foullon et al., 2008)
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C1 electron
C1 ion
density
temperature
velocity
magnetic field
2.5 hours
Cluster event on 20 Nov 2001
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TotalPvv
)(
Total-P perturbation in the vortex
streamline
Force balance
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Wavelet spectrum of Total-P
• Dominant-mode period ~200 s (Wavelength ~6 Re)• Power also at ~400 s: Beginning of vortex pairing?
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C3
Vortex structurefrom Grad-Shafranov-like (GS-like) reconstruction of
streamlines (Sonnerup et al., 2006; Hasegawa et al., 2007)
• A GS-like eq. for “stream” function is solved, as a spatial initial value problem.
Assumptions: MHD, 2D, time-independent, & B || z.
• Two vortices within one dominant-mode wavelength.
Breakup of a parent MHD-scale vortex (cascade)?
Dominant-mode wavelength ~6 Re
TotalPvv
)(
The KHI seen by Cluster was fully in a nonlinear phase, characterized by coalescence/breakup (inverse-cascade/cascade) of the vortices.
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A cascade process at flank magnetopause (Takagi et al., JGR, 2006)
• In the flanks, KHI can grow only near equator.
• A dominant KH mode grows and bends field lines.
• Magnetic tension of the bent field lines deforms the parent KH vortex, or create smaller vortices within.
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closest to Earth0600 UT
1000 UT
X (sunward)
Y (dusk)
THEMIS string-of-pearls observation of a dayside MP boundary layer (BL)@16 MLT
8 June 2007
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THEMIS obs. of a dayside BL
• Surface waves activity with 1-2 min period
• Simultaneous BL encounters by 2-4 SC, at several times.
• SC separation along MP normal ~0.5 Re.
↓BL width ~0.5 Re40 min
closest to Earth
Eriksson et al., JGR, 2009
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• Bipolar BN, at BL-to-sheath transitions, i.e., at the sunward-side edge of the surface wave.
Bipolar BN (FTEs) on the surface wave
BN
80 min
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streamline
B-field
streamline
Recovery of 2D MHD structureEriksson et al., JGR, 2009
• Magnetic island & small vortex between two large-scale vortices
• Local reconnectionleading to the magnetic island formation
sheath side
Plasma sheet
N
T
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Nakamura et al., JGR, 2008
Interpretation of the THEMIS event
• Thin current sheet forms at the sunward-facing edge of KH waves, where the CS is compressed by vortex flow, and it may become subject to reconnection.
• KH-induced reconnection can form a magnetic island.• Open question: can it lead to large-scale or efficient
plasma transport?
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Summary• Coalescence and breakup of KH vortices
(inverse-cascade & cascade) are beginning at ~19 MLT (just behind the terminator).
Evidence of nonlinear KHI development.
• Magnetic island formation preferentially at the sunward-facing edge of KH waves.
It most likely resulted from local reconnection at the current sheet thinned by vortex flow.
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As a spatial initial value problem,V, n, & T recovered
Assumptions: MHD, 2D, time-independent, & B along z axis
GS-like equation for the stream function
GS-like reconstruction of streamlines(Sonnerup et al., 2006; Hasegawa et al., 2007)
TotalPvv
)(
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• Spatial initial value problem• Assumptions: MHD, d/dt =0, 2D, & B along invariant axis z.
Dominant-mode wavelength ~6 Re
Vortex structurefrom Grad-Shafranov-like reconstruction of
streamlines (Sonnerup et al., 2006; Hasegawa et al., 2007)
C1
C3
• Two vortices within one dominant-mode wavelength.
Breakup of a parent MHD-scale vortex (cascade)?
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Fluctuation in the dayside boundary
• Magnetic fluctuations had a period similar to that of the KH waves.
Geotail Cluster
The KHI was excited by the mechanism that generated the magnetic fluctuations.
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