Jovian Sbursts generation by Alfvn waves

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Jovian S-bursts generation by Alfvén waves
Sébastien Hess1,2 F. Mottez1 P.Zarka2 1
LUTH - 2 LESIA Observatoire de Paris - Meudon
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Io-Jupiter interaction

• Torus period Jupiters Period 9 hours 55.5
  min. Io is moving relative to the plasma
• Ios obital period 42 hours 27.5 minutes
• Electric field generation E -v?B
• Induced currents transported by Alfvén waves
  ??electron acceleration toward Jupiter

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Io-Jupiter interaction

• The Io-Jupiter interaction generate a strong
  auroral activity in UV and IR.
• With a counterpart in decametric radio emissions.
• Theory These emissions due to maser
  instability, need an electron energy gt keV.
• At Io the electron temperature is 5 eV. Colder
  near Jupiter.
• gt Strong electron acceleration by the Io-Jupiter
  interaction is  required to explain the
  decametric radiation.

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Millisecond bursts

• Intense, discrete
• Cyclotron-MASER emissions near the local electron
  cyclotron frequency gtFrequency signature of
  altitude.
• Drift in the time frequency plane
• f proportional to B gt source moving away from
  Jupiter
• Adiabatic motion of the emitting electrons
  Ellis,65 Zarka et al.,96 Hess et al.,2007
• Quasi periodic 15 Hz.

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Questions

• Maser emission implies an appropriate electron
  distribution function (including acceleration)
  how are electron accelerated ?
• how their distribution function evolve along the
  Io-Jupiter flux tube ?
• Can these distributions actually generate maser
  instability ?
• Can we reproduce the features observed in the
  dynamic spectrum ?
• Can we reproduce their pseudo-periodic
  oscillations ?

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Conclusion

• Kinetic Alfvén wave generate electron beams...
• ...which move adiabatically (consistent with the
  S-burst source motion)...
• ... and generate bursty radio emissions
  (S-bursts) by loss-cone driven cyclotron-maser
  instability
• Bursts repeat periodically with the Alfvén wave
  period

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Cyclotron Maser Instability

• Emissions near the local electron cyclotron
  frequency
• Resonant instability
• ?-??c-k//v//0
• gt resonance circle
• Amplification if positive perpendicular gradient
  of the distribution along the resonance circle.
• Two energy sources
• Loss-cone
• Shell

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Millisecond bursts

• Intense, discrete
• Cyclotron-MASER emissions near the local electron
  cyclotron frequency
• Drift in the time frequency plane
• f proportional to B gt source moving away from
  Jupiter
• Adiabatic motion of the emitting electrons
• Quasi periodic

Alfvén waves ?
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Kinetic Alfvén waves

• MHDPropagation along the Io Flux Tube (kk??)
• But electric field perpendicular to the magnetic
  field lines
• gt no parallel acceleration
• kinetic Alfvén wave if oblique propagation (k? ltlt
  k??) then, a parallel electric field exists
• gt parallel acceleration is possible

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Simulation

• Electrons are injected at Io boundary and exit
  form the box at the both boundaries
• The electron distribution in Io Flux Tube is
  simulated with a test particle code
  
• When the flux tube is filled with particles, a
  kinetic Alfvén wave is (analytically) injected
  and propagated. The associated parallel electric
  field is computed.
  
• Cyclotron-Maser instability linear growth rate
  are computed at several times and altitudes

Alfvén waves
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imposed Alfvén wave parallel electric field

• Chossen freq. 5 Hz,
• amplitude B 0.01 G
• injected toward Jupiter during 3s.
  
• The kinetic Alfvén wave parallel electric field
  reaches its maximum value at 1.5-2 RJ
• Negligible in the emission region
• from Lysak and Song 2003.

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result distribution function

• The abrupt decrease of the Alfvén wave electric
  field (?0,8 Rj) accelerate planetward electron
  beams...
• ... which generate arcs due to magnetic field
  lines convergence.
• Arcs are partially reflected
• Electrons beams move adiabatically in the
  emission region

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result Dynamic spectrum/loss-cone

• From second 2 to 5
• wave passing local disturb of the distribution
• From second 5
• drifting structures electron beam in adiabatic
  motion.
• Drift 17 Mhz/s
• consistent with the S-bursts

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result Quasi periodicity of the bursts

• Fourier transform of the simulated dynamic
  spectra over time show a periodicity of 5 sec.
• FT of a real dynamic spectra show the same
  profile, with a periodicity of 15 sec.

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Dynamic spectrum shell

• No S-bursts-like structures
• both upward and downward beams generate emission
• Growth rate higher than for loss-cone driven
  instability...
• ... but the linear equation is not accurate in
  the shell case (need of relativistic dispersion
  equation)

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Conclusion

• Kinetic Alfvén wave generate electron beams...
• ...which move adiabatically (consistent with the
  S-burst source)...
• ... and generate bursty radio emissions
  (S-bursts) by loss-cone driven cyclotron-maser
  instability
• Bursts repeat periodically with the Alfvén wave
  period