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

Auroral S/C observations - steep gradient density cavities - related phenomena (Alfvén waves) 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 2 1

Lundin et al Cavity events in VIKING data Hilgers et al Density : n min ~ 0.25 n 0 Gradient size : ~2 km i.e. a few ion Larmor radius, i.e. a few c/  pe. => Strong density gradients

Chaston et al., 2000 Zoom on a cavity cold hot Cavity events in FAST data Density : n min ~ 0.1 n 0 Gradient size : ~2 km Alfvén waves

Observations of deep cavities by FAST the cold plasma has been completely expelled plasma instrument Langmuir probe Factor 10 FAST crossed many deep cavities (n/n 0 ~ ) in the altitude range km Factor 20 Deep cavities are ubiquitous in the auroral zone from FREJA, FAST, VIKING, to CLUSTER (~5R e ) altitudes.

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

2.5D PIC simulations Alfvén waves + perpendicular density gradients Processes on the gradient the AW polarization drift moves ions  space charge  E// forms on a large scale (λ A )  electron motion  plasma instabilities front torsion Génot et al Génot et al Génot et al Direction  to B 0 Density

Plasma instabilities : Buneman instability V drift >> V the Beam-plasma instability V the-beam /V drift-beam << (n e-beam /n e ) 1/3 beam V drift V the V drift-beam V the-beam During the simulation, electron distribution functions on density gradients evolve and lead to different instabilities

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

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

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.

Stochastic ion acceleration 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 : E  /B 0 > ω ci /k  Chaston et al Numerically, for ω/ω ci as low as 0.05 stochastic behaviour is obtained for α=mk  2 Φ 0 /qB 0 2 ≥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. regime coherent stochastic 4α4α 4α4α

E-field structure in the cavity E-field profile across the magnetic field 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) Regions where α≥0.8 using k  2 Φ 0 =dE  /dx

Stochastic ion acceleration References : - Karney 1978, Karney & Bers McChesney et al. 1987, lab related - Stasiewicz et al FREJA related - Chen et al Chaston et al 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 Reitzel & Morales localized field - Ram et al Strozzi et al. 2003

Transverse acceleration of ions Thermal ion Initial orbit E-field profile across the gradientMean perpendicular kinetic energy 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.

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

Conclusion Alfvén 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