Transcript Parallel acceleration and structuration of the electric

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:
a complex chain of
processes still not fully
described.
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 %
Alfvén Waves
Large scale structures bringing energy
from other regions of the magnetosphere
Alfvén dE/dB~VA
Compressional dn/n0~0.2
Amplitude dB ~60 nT
dB/B0z~0.01
Strong Poynting flux
Transverse size ~ c/wpe~ri
SKAW, Freja [Louarn et al., 1994]
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)
• b << me/mi, on néglige la pression
2V 2
k
z
A
w 2
1(kxc/w pe)2
Il existe un champ électrique parallèle à B
2
k
x
k
z
c
Ez  E// 
2
Ex E kx2c2w pe
L’ onde d’Alfvén inertielle
Mais 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 ?
2
k
x
k
z
c
Ez  E// 
2
Ex E kx2c2w pe
Deep auroral density depletions
Deep cavities: nmin ~ 0.1 n0
Size of the gradients ~2 km
i.e. a few ion Larmor radius,
i.e. a few c/wpe.
=> Strong density gradients
Viking, [Hilgers et al., 1992]
The basic principle :
Alfvén waves + perpendicular density gradients
parallel propagation
at VA (E//=0)
B0z
+
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
Planar wave front
Oblique wave front
Oblique wave front => E//
=> energy from wave to plasma
=> acceleration and turbulence
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
Ez=E//
Haute densité n=1
Ne
Cavité n=1/3
Ex
204.8
B0z
x
axes
dB/B0z=0.1
z
La cavité est stable
Pas de champ électrique associé
Case 2 : Alfvén wave alone
Bx
Ez=E//
Ne
Ex
L’onde se propage le long du champ magnétique ambiant
polarisation circulaire (ici gauche, pourrait être droite ou lin.)
Pas de champ électrique parallèle
Alfvén wave on a plasma cavity
B0z
Bx
Ez=E//
VA
12.8
Ne
x
Ex
204.8
axes
z
Space
Earth
E//(t) upon a
density gradient
Large scale fields
Beam-plasma instability
Buneman instability
time
Large scale fields
of the inertial Alfvén wave
Z (along B)
Particles
Ez(z,x)
Lower gradient
<Ez>(z) over the
lower channel
Fe(z,vz) over the
lower channel
z
Large scale electric
field and
electron halo
Weak (oblique Alfvén ) E//
over large distances
Electron parallel heating :
« halo » i.e.
tail in the distribution function
Assymetry : propagation
of the Alfvén wave / electron velocities
Runaway
electron
Faster electrons from the
halo espace first and create
an electron beam.
E// over large distances
halo
runaway
Beam
dynamics
Finite beam in an
inhomogeneous
plasma.
Backward slow vortices
(Buneman)
Fast vortices (beam-plasma)
electron-beam
Buneman
Electron
holes
Spread velocity distribution
Electron holes in both directions
Remaining localized beams
beam
forward
backward
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.
Conclusion
Alfvén wave along a density gradient :
a cascade of events leading to acceleration
and turbulence
Parallel electric fields: large scale, then small scale, then large scale, etc.
Acceleration: halos, runaway electrons, beams
Turbulence: structuration of beams as series of (z,Vz) vortices
Turbulence: various kind of coherent structures, electron holes
Prefered 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.
publications
Alfvén 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.