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

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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
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• Auroral S/C observations
• steep gradient density cavities
• related phenomena (Alfvén waves)

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• Modelization of the interaction Alfvén
  wavescavity
• Results on
• parallel electric field formation
• electron acceleration
• ion heating
• coherent electrostatic structures
• cyclic scenario of acceleration/dissipation and
  plasma/field reorganization

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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. gt Strong density gradients
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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
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Observations of deep cavities by FAST
FAST crossed many deep cavities
(n/n00.1-0.05) 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.
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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

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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 (?A) ? electron motion ? plasma
instabilities
Génot et al. 1999 Génot et al. 2000 Génot et al.
2001
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During the simulation, electron distribution
functions on density gradients evolve and lead to
different instabilities
Plasma instabilities
Buneman instability Vdrift gtgt Vthe
Beam-plasma instability Vthe-beam/Vdrift-beam ltlt
(ne-beam/ne)1/3
Vthe
Vthe-beam
beam
Vdrift
Vdrift-beam
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Parallel electric field in the (X,Z) space
Parallel electron phase space
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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)
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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.
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Stochastic ion acceleration
4a
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
4a
stochastic
E?/B0 gt ?ci/k?
Chaston et al. 2004
Numerically, for ?/?ci as low as 0.05 stochastic
behaviour is obtained for amk?2F0/qB020.8. 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.
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E-field structure in the cavity
E-field profile across the magnetic field
Regions where a0.8 using k?2F0dE?/dx
The differential propagation in the cavity leads
to the torsion of the wave front.
The stochastic criterion a0.8 is satisfied in
very localized regions (density gradients)
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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
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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.
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Stack plots over ?A/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
Ne1
Px
Ne0.2
Ne0.5
?A/4 (direction // to B)
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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