Alfvén wave interaction with inhomogeneous plasmas : acceleration and turbulence.

1. Alfvén wave interaction with inhomogeneous plasmas : acceleration and turbulence. F. Mottez, V. Génot, P. Louarn

2. What ? Electron accélération toward the Earth (10 000 km) How ? An Alfvén wave + A density gradient

3. 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 %

4. Alfvén Waves Large scale structures bringing energy from other regions of the magnetosphere AlfvéndE/dB~VA Compressionaldn/n0~0.2 AmplitudedB ~60 nT dB/B0z~0.01 Strong Poynting flux Transverse size ~ c/wpe~ri SKAW, Freja [Louarn et al., 1994]

5. 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 …

6. L’ onde d’Alfvén inertielle • Théorie bi-fluide, centres guides (dérive de polarisation des ions) • b << me/mi, on néglige la pression Il existe un champ électrique parallèle à B

7. 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 ?

8. 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]

9. VA = B/(n1/2) higher in low density region parallelpropagation at VA (E//=0) + The basic principle :Alfvén waves + perpendicular density gradients high density small VA grad n B0z low density large VA grad n small VA high density Planar wave front Oblique wave front Oblique wave front => E// => energy from wave to plasma => acceleration and turbulence

10. 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.

11. Haute densité n=1 12.8 Cavité n=1/3 204.8 x axes B0z dB/B0z=0.1 z Case 1 : cavity alone Bx Ne Ez=E// Ex La cavité est stable Pas de champ électrique associé

12. Case 2 : Alfvén wave alone Bx Ne Ez=E// 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

13. Space Earth Alfvén wave on a plasma cavity B0z Ez=E// Bx VA 12.8 Ex Ne x 204.8 axes z

14. E//(t) upon a density gradient Large scale fields Beam-plasma instability Buneman instability Large scale fields of the inertial Alfvén wave time Z (along B)

15. Particles Ez(z,x) Lower gradient <Ez>(z) over the lower channel Fe(z,vz) over the lower channel Vz z

16. Large scale electric field andelectron 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

17. Runaway electron Faster electrons from the halo espace first and create an electron beam. E// over large distances halo runaway

18. Beam dynamics Finite beam in an inhomogeneous plasma. Backward slow vortices (Buneman) Fast vortices (beam-plasma) electron-beam Buneman

19. Electron holes Spread velocity distribution Electron holes in both directions Remaining localized beams beam forward backward

20. 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.

21. 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.

22. 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.