Transcript third alfven conference aout 2004

Numerical simulations of
wave/particle interactions in
inhomogeneous auroral plasmas
Vincent Génot (IRAP/UPS/CNRS, Toulouse)
F. Mottez (LUTH/CNRS, Meudon)
P. Louarn (IRAP/UPS/CNRS, Toulouse)
C. Chaston (SSL, Berkeley)
Observations of deep cavities by Viking
• Depth of the cavities: nmin ~ 0.1 n0
• Size of the gradients : ~a few km
i.e. a few c/wpe.
=> Strong density gradients
Density cavity related physics have initially
been investigated in connection with AKR
generation (Louarn et al., 1990)
Hilgers et al., 1992
Observations of deep cavities by FAST
FAST crossed many deep cavities (n/n0~0.1-0.05)
in the altitude range 1500-4000 km
Langmuir
probe
Factor 20
plasma
instrument
Factor 10
the cold plasma
has been completely expelled
Cluster
observations
For more Cluster
observations of
density cavities,
see Marklund et al.
Electric field
S/C potential ~ density
Upflowing ions
Density cavity
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FAST Data
Electric turbulence
}
}
Transverse ion acceleration
Ion outflow
Fiel-aligned electron
acceleration
Chaston et al., 2007
Wave identification in the density cavity
Alfvén wave
Small perpendicular scales
inertial
range
dissipation
range
Chaston et al., 2007
• What is the mechanism for the formation of small
perpendicular scales ?
•What is the effect of the associated parallel electric
field ?
•Can it be maintained on sufficiently long time ?
The basic principle :
Alfvén wave + perpendicular density gradient
Initial parallel propagation
at VA (E//=0)
B0
+
VA = B/(n1/2) higher in low
density region
grad n
high density small VA
grad n
low density large VA
high density small VA
Oblique wave front
 k≠0
 E//
Analytic study
Propagation of Alfvén waves in a density cavity : analytic model 1/2
magnetosphere
ionosphere
cavity
•Alvénic pulse propagation
•density cavity
•field line convergence
E
cavity
Alfvén velocity profile
Time
cavity
Wave front torsion due to differential VA
Génot et al., 1999
Propagation of Alfvén waves in a density cavity : analytic model 2/2
E//
E
E//
E
E//
magnetosphere
This 2D model assumes :
-electroneutrality
-perpendicular ion motion
(polarization drift)
-parallel electron motion
Strong E// are formed on
density gradients (about
1% of the incoming field)
Génot et al., 1999
ionosphere
Self consistent PIC simulations
Particle In Cell simulations
• Full ion dynamics
• Electron guiding centre dynamics
• Electromagnetic
Mottez et al. 1998
• 2D in space, 3D in velocities and electromagnetic fields
• Periodic boundary conditions
• Simulation box : 204.8 X 12.8 (c/wpe)2 (or multiple)
• Reduced mass ratio: mi/me=100, 200, 400
• Strong ambient magnetic field: wce/ wpe = 4 (auroral zone)
• Ordering: c/wpe~ri for the auroral zone (~1 km)
Génot et al., 2000, 2001, 2004
A simple situation :
infinite cavity and wave
VA
k=0
ne
 to B
ne/4
// to B
Density map
Génot et al., 2004
Stack plot of E//(z,t)
integrated on a
density gradient
•Large scale fields
•Beam-plasma instability
•Buneman instability
•Large scale inertial Alfvén wave
time
•Formation of small transverse scales
Z (along B)
FAST observations
Chaston et al., 2006
FAST observations show that
1/ the wave power maximises in
the centre of the cavity
2/ large field aligned Poynting
flux on the wall of the cavity
(directed mostly Earthward)
3/ the wave focuses to the centre
of the cavity (converging
transverse Poynting flux)
The inward focused Poynting flux indicates that the wave group velocity is
convergent on the cavity. These observations suggest that wave refraction on
the cavity walls leads to the focusing of Alfvén wave energy within the cavity,
therefore increasing the wave turbulence inside the cavity.
Simulation results
Chaston et al., 2006
Converging
transverse Poynting
flux directed inside
the cavity
A more realistic situation :
infinite cavity and pulse
VA
k=0
ne
 to B
ne/4
// to B
Density map
Simulation: Alfvén pulse in a cavity
•4096 X 128
•mi/me=400
•ΔN/N=0.8
•Δ B/B=0.032
Time
E//
Direction along B
Mottez & Génot, 2011
Simulation: Alfvén pulse in a cavity
•4096 X 128
•mi/me=400
•ΔN/N=0.8
•Δ B/B=0.032
V//
(along B)
vthe_beam=0.04
vdrift_beam=0.69
ne_beam=0.026
ne=1.21
Beam/plasma
instability
Vthe_beam/Vdrift_beam << (ne_beam/ne)1/3
Mottez & Génot, 2011
Simulation: Alfvén pulse in a cavity
Acceleration in the same direction as the wave propagation is favored
Energy flux
↓
Energy flux
electrons
protons
↓
↑
↑
↓
↓
↑
↑
Mottez & Génot, 2011
Identification of the acceleration process
Inertial effect: ion polarization drift (~mi/me) and electron inertia (~1/me)
E//max
mi/me=400
mi/me=200
mi/me=100
Mottez & Génot, 2011
An even more realistic situation :
localized cavity and pulse
VA
k=0
Density map
 to B
ne/4
ne
// to B
Unpublished work
1D cut on the
density gradient :
Perpendicular
electric field
Time
ne=0.5
ne=1.15
The AW enters
the plasma cavity
VA
E max
Density cavity contours
ne=0.75
E
1D cut on the
density gradient :
Parallel
electric field
V//e
Time
+10%
E// formation
Electron acceleration
Density cavity contours
E max
ne=0.75
E//
// to B
Cavitation/filamentation
mechanism
Pre-existing depletion
Wave focusing
Converging Poynting flux
Density cavitation
Wave amplitude
enhancement
and
filamentation
E//
Plasma outflow/acceleration
in the cavity
Perspectives
•Real mass ratio and quantification of the acceleration process
•Use multi-spacecraft observations by Cluster to constrain the
parallel extension of the cavity w.r.t the size of the Alfvén pulse
•3D aspects: use of a Landau fluid code with electron inertia
(Borgogno et al. 2009)