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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. 1. Auroral S/C observations steep gradient density cavities - PowerPoint PPT Presentation
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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)
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/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 cavityE-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