Boundaries in the auroral region ---

Small scale density cavitiesand

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. 1990Cavity events in VIKING dataHilgers et al. 1992

Density : nmin ~ 0.25 n0

Gradient size : ~2 kmi.e. a few ion Larmor radius, i.e. a few c/pe.

=> Strong density gradients

Chaston et al., 2000

Zoom on a cavity

coldhot

Cavity events in FAST data

Density : nmin ~ 0.1 n0

Gradient size : ~2 km

Alfvén waves

Observations of deep cavities by FAST

the cold plasmahas been completely expelled

plasma instrument

Langmuir probe

Factor 10

FAST crossed many deep cavities (n/n0~0.1-0.05)in the altitude range 1500-4000 km

Factor 20

Deep cavities are ubiquitous in the auroral zone from FREJA,

FAST, VIKING, to CLUSTER (~5Re) 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 simulationsAlfvén waves + perpendicular density gradients

Processes on the gradientthe AW polarization drift moves ions space charge E// forms on a large scale (λA) electron motion plasma instabilities

front torsion

Génot et al. 1999Génot et al. 2000Génot et al. 2001

Direction to B0

Density

Plasma instabilities :

Buneman instabilityVdrift >> Vthe

Beam-plasma instabilityVthe-beam/Vdrift-beam << (ne-beam/ne)1/3

beamVdrift

Vthe

Vdrift-beam

Vthe-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)

tim

e

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/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.

regime

coherent

stochastic

4α

4α

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

2Φ0=dE/dx

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

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// Px=(ExB)x

E//

Ne=1

Ne=0.2

Ne=0.5

λA/4 (direction // to B)

direction to B

direction

to B

Stack plots over λA/4

Px

dNe/dx and Px

correlation factor = -0.88

The Alfvén waveis 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 cycleAcceleration : electrons, TAIPreferred direction of acceleration: direction of Alfvén wave propagationTurbulence in phase space : electron beams structured as vorticesTurbulence 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