Boundaries in the auroral region --- Small scale density cavities and associated processes ---

1. Boundaries in the auroral region ---Small scale density cavitiesandassociated processes--- Vincent Génot (CESR/CNRS) C. Chaston (SSL) P. Louarn (CESR/CNRS) F. Mottez (CETP/CNRS) Abisko, Sweden, December 1998

2. 1 • Auroral S/C observations • steep gradient density cavities • related phenomena (Alfvén waves) 2 • Modelization of the interaction Alfvén waves+cavity • Results on : • parallel electric field formation • electron acceleration • ion heating • coherent electrostatic structures • cyclic scenario of acceleration/dissipation and plasma/field reorganization

3. Cavity events in VIKING data Lundin et al. 1990 Hilgers et al. 1992 Density : nmin ~ 0.25 n0 Gradient size : ~2 km i.e. a few ion Larmor radius, i.e. a few c/wpe. => Strong density gradients

4. Cavity events in FAST data Alfvén waves cold Density : nmin ~ 0.1 n0 Gradient size : ~2 km hot Zoom on a cavity Chaston et al., 2000

5. 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 Factor 10 plasma instrument the cold plasma has been completely expelled Deep cavities are ubiquitous in the auroral zone from FREJA, FAST, VIKING, to CLUSTER (~5Re) altitudes.

6. The auroral density cavity is a magnetospheric boundary • Cavities are regions : • of tenuous hot plasmas (dense cold outside) • where turbulence is present (quiet outside) • - where waves are emitted (-) • The boundary (=density gradient) itself is an ideal location for : • non homogeneous E-field • formation of E// • parallel electron acceleration • transverse ion acceleration

7. 2.5D PIC simulationsAlfvén waves + perpendicular density gradients front torsion Density Direction  to B0 Processes on the gradient the AW polarization drift moves ions  space charge  E// forms on a large scale (λA)  electron motion  plasma instabilities Génot et al. 1999 Génot et al. 2000 Génot et al. 2001

8. During the simulation, electron distribution functions on density gradients evolve and lead to different instabilities Plasma instabilities : Buneman instability Vdrift >> Vthe Beam-plasma instability Vthe-beam/Vdrift-beam << (ne-beam/ne)1/3 Vthe Vthe-beam beam Vdrift Vdrift-beam

9. Parallel electric field in the (X,Z) space Parallel electron phase space

10. E//(z,t) on a density gradient 4 Large scale fields 3 Beam-plasma instability 2 Buneman instability 1 Large scale inertial Alfvén wave Cascade toward small scales time Génot et al. 2004, Ann. Geophys. Z (along B)

11. 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.

12. Stochastic ion acceleration 4α The ion motion in the electrostatic wave field may become stochastic if the displacement of the ion guiding center due to the polarization drift over one wave period is similar to, or greater than, the perpendicular wavelength : coherent regime 4α stochastic E/B0 > ωci/k Chaston et al. 2004 Numerically, for ω/ωci as low as 0.05 stochastic behaviour is obtained for α=mk2Φ0/qB02≥0.8. In this regime a larger part (than in the coherent regime) of the velocity space can be explored by the particles enabling them to reach large velocities.

13. E-field structure in the cavity E-field profile across the magnetic field Regions where α≥0.8 using k2Φ0=dE/dx The differential propagation in the cavity leads to the torsion of the wave front. The stochastic criterion α≥0.8 is satisfied in very localized regions (density gradients)

14. Stochastic ion acceleration References : - Karney 1978, Karney & Bers 1977 - McChesney et al. 1987, 1991 -- lab related - Stasiewicz et al. 2000 -- FREJA related - Chen et al. 2001 - Chaston et al. 2004 -- FAST related But “real” electric field usually present a spectrum of k which complicates this ideal scenario. However adding multiple modes or considering a localized field generally lowers the threshold for stochasticity. References : - Lysak et al. 1980, Lysak 1986 - Reitzel & Morales 1996 -- localized field - Ram et al. 1998 - Strozzi et al. 2003

15. Transverse acceleration of ions E-field profile across the gradient Mean perpendicular kinetic energy Thermal ion Initial orbit k≠0 k=0 Transverse ion acceleration actually occurs in the cavity due to the perpendicular structure of the E-field although the classical stochastic criterion is satisfied only locally. We speculate that the multi-modes nature of the field (i.e. lower stochastic threshold) is responsible for the acceleration.

16. Stack plots over λA/4 dNe/dx and Px correlation factor = -0.88 dNe/dx The Alfvén wave is focused into the cavity Soon : comparison with FAST data Chaston & Génot, 2005 Px=(ExB)x E// direction  to B E// direction  to B Ne=1 Px Ne=0.2 Ne=0.5 λA/4 (direction // to B)

17. ConclusionAlfvén wave interaction with density gradients a cascade of events leading to acceleration and turbulence • Parallel electric fields : large scales to small scales, EM to ES, in a cycle • Acceleration : electrons, TAI • Preferred direction of acceleration: direction of Alfvén wave propagation • Turbulence in phase space : electron beams structured as vortices • Turbulence as electrostatic coherent structures : electron holes, DL Does not require initial inertial or kinetic AW, or a permanent beam Cavity structure : the density gradients remain ~ stable. The cavity is not destroyed and is 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 by saturating the electron acceleration process