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Excitation and Multi-scale development of Kelvin-Helmholtz (KH) waves at the Earth’s magnetopause. H. Hasegawa (1), A. Retin ò(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) - PowerPoint PPT Presentation
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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
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.
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?
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)
C1 electron
C1 ion
density
temperature
velocity
magnetic field
2.5 hours
Cluster event on 20 Nov 2001
TotalPvv
)(
Total-P perturbation in the vortex
streamline
Force balance
Wavelet spectrum of Total-P
• Dominant-mode period ~200 s (Wavelength ~6 Re)• Power also at ~400 s: Beginning of vortex pairing?
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.
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.
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
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
• 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
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
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?
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.
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
)(
• 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)?
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.