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Alfvén wave interaction with inhomogeneous plasmas : acceleration and turbulence. F. Mottez, V. Génot, P. Louarn. What ? Electron accélération toward the Earth (10 000 km). How ? An Alfvén wave + A density gradient. The auroral particle acceleration: - PowerPoint PPT Presentation
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Alfvén wave interaction with inhomogeneous plasmas :
acceleration and turbulence.
F. Mottez, V. Génot, P. Louarn
How incoming energy from distant regions of the magnetosphere (for example, in the form of the Poynting flux of Alfven waves) can be converted into the kinetic energy of accelerated particles
•up to KeV energies •Efficiency larger than some 10 %
The auroral particle acceleration: a complex chain of processes still not fully described.
Large scale structures bringing energy from other regions of the magnetosphere
Alfvén Waves
SKAW, Freja [Louarn et al., 1994]
Alfvén E/B~VA
Compressional n/n0~0.2
Amplitude B ~60 nTB/B0z~0.01
Strong Poynting flux
Transverse size ~ c/pe~i
Accélérer avec le E// d’une onde d’Alfvén
Une onde MHD (Alfvén ou magnétosonique) ne porte pas de E//
Mais si on s’écarte un peu de la MHD :Onde d’Alfvén inertielle …
L’ onde d’Alfvén inertielle• Théorie bi-fluide, centres guides (dérive de polarisation des ions) • << me/mi, on néglige la pression
Il existe un champ électrique parallèle à B
222//
2
pex
zx
x
z
ckckk
EE
EE
2
222
)/(1 pex
Az
ckVk
L’ onde d’Alfvén inertielleMais il faut du kx. (une vitesse de phase oblique).La vitesse de groupe est assez parallèle. Origine de kx loin de la zone d’accélération ?
Mais alors, pourquoi accélération localisée ?
Quelle est l’origine du kx ?
222//
2
pex
zx
x
z
ckckk
EE
EE
Deep auroral density depletions
Viking, [Hilgers et al., 1992]
Deep cavities: nmin ~ 0.1 n0
Size of the gradients ~2 kmi.e. a few ion Larmor radius, i.e. a few c/pe.
=> Strong density gradients
The basic principle :Alfvén waves + perpendicular density gradients
parallel propagation at VA (E//=0)
+ VA = B/(n1/2) higher in low density region
B0z
high density
low density
high density
Oblique wave frontPlanar wave front
Oblique wave front => E// => energy from wave to plasma => acceleration and turbulence
small VA
small VA
large VA
grad n
grad n
In the auroral zone, VA > Vte
• This is not a resonnant process. The wave goes (initially) much faster than most of the particles.
• Because of the long wavelength of the wave, the particles see an electric field for a few milliseconds. This is enough for acceleration.
Case 1 : cavity alone
Bx
Ne
Ez=E//
Ex
La cavité est stablePas de champ électrique associé
B/B0z=0.1B0z
12
.8
204.8
Cavité n=1/3
Haute densité n=1
z
x axes
Case 2 : Alfvén wave alone
Bx
Ne
Ez=E//
Ex
L’onde se propage le long du champ magnétique ambiantpolarisation circulaire (ici gauche, pourrait être droite ou lin.)Pas de champ électrique parallèle
E//(t) upon a density gradient
Large scale fields
Beam-plasma instability
Buneman instability
Large scale fields of the inertial Alfvén wave
Z (along B)
tim
e
Weak (oblique Alfvén ) E// over large distances
Electron parallel heating : « halo » i.e. tail in the distribution function
Large scale electric field and
electron halo
Assymetry : propagationof the Alfvén wave / electron velocities
Runaway electron
E// over large distances
halo
runaway
Faster electrons from the halo espace first and create an electron beam.
Beam dynamics
Finite beam in an inhomogeneous plasma.
Backward slow vortices(Buneman)
Fast vortices (beam-plasma)
Buneman
electron-beam
Electron holes
Spread velocity distribution
Electron holes in both directions
Remaining localized beams
backward
forwardbeam
Wave and electron energies over 4 Alfvén periods
The energy exchange between the Alfvén wave and the electrons occurs when there are no coherent structures : before their formation (growth of the beam) or after their destruction.
ConclusionAlfvén wave along a density gradient : a cascade of events leading to acceleration and turbulenceParallel electric fields: large scale, then small scale, then large scale, etc.Acceleration: halos, runaway electrons, beamsTurbulence: structuration of beams as series of (z,Vz) vorticesTurbulence: various kind of coherent structures, electron holesPrefered direction of acceleration: direction of Alfvén wave .The plasma cavity is not destroyed : ready for the next Alfvén wave train.
Role of the coherent structures : they contribute to reorganize the plasma under the influence of a large scale parallel electric field; they saturate the electron acceleration process.
Geophysical relevance of this process :Could explain the small scale structuration of the discrete auroras (100 m) and the high level of turbulence observed around the auroral plasma cavities.
publicationsAlfvén wave interaction with inhomogeneous plasmas : acceleration and energy cascade toward small scalesV. Genot, P. Louarn, F. Mottez, Annales Geophysicae, 2004.
Electron acceleration by Alfvén waves in density cavities, Génot et al., J. Geophys. Res. 105, 2000.
Fast evolving spatial structure of auroral parallel electric fields, Génot et al., J. Geophys. Res. 106, 2001.
A study of the propagation of Alfvén waves in the auroral density cavities, Génot et al., J. Geophys. Res. 104, 1999.