Solar%20wind%20turbulence%20from%20radio%20occultation%20data

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Solar wind turbulence from radio occultation
data

• Chashei, I.V.
• Lebedev Physical Institute, Moscow, Russia
• Efimov, A.I.,
• Institute of Radio Engineering Electronics,
  Moscow, Russia
• Bird, M.K.
• Radio Astronomical Institute, Univ. Bonn, Bonn,
  Germany

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TURBULENCE

• Turbulence is a permanent property of the solar
  wind.
• Fluctuations spectra of B, N, V cover many
  decades in wavenumbers / frequencies.
• Formed flow R gt 20 RS , in situ
  radiooccultation data.
• Acceleration region R lt 10 RS, no in situ data.
• Below we concentrate mainly on Galileo and
  Ulysses spacecraft data.

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GEOMETRY OF CORONAL RADIO OCCULTATION
EXPERIMENT
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OBSERVATIONAL DATA

• SPACECRAFT GALILEO (1994-2002) AND ULYSSES
  (1991-1997)
• HIGH STABILITY RADIO SIGNALS AT S-BAND (2295 ?Hz)
  
• GROUND BASED NASA-DSN TRACKING STATIONS
• GOLDSTONE (DSS 14)
• CANBERRA (DSS 43)
• MADRID (DSS 63)
• MEASUREMENTS OF FREQUENCY FLUCTUATIONS
• SAMPLING RATE 1 Hz
• RECORDS AT INDIVIDUAL STATIONS ?
• TEMPORAL POWER SPECTRA OF FREQUENCY FLUCTUATIONS
• CROSS CORRELATION OF OVERLAPPING RECORDS ?
• VELOCITY OF THE DENSITY IRREGULARITIES
• SOLAR OFFSET R 7 R? lt R lt 80 R?

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EXAMPLE (ULYSSES) OF FREQUENCY FLUCTUATION RECORD
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TEMPORAL POWER SPECTRA OF THE FREQUENCY
FLUCTUATIONS

• Typical temporal spectra are power law
• Power law interval is bounded by the frequency of
  the turbulence outer scale at low frequencies and
  the noise level at high frequencies
• Power law spectral index of the temporal
  frequency fluctuation spectrum ? is related to
  the power law index of the 3D spatial turbulence
  spectrum ? by the equation ? ?-3

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FREQUENCY FLUCTUATION POWER SPECTRA SOME
EXAMPLES
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CROSS-CORRELATION FUNCTIONFREQUENCY FLUCTUATIONS
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RADIAL EVOLUTION OF THE SPECTRAL INDEX (LOW
HELIOLATITUDES)
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RADIAL EVOLUTION OF THE SPECTRAL INDEX (HIGH
HELIOLATITUDES, R 22-30 R?)
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FRACTIONAL LEVEL OF DENSITY VARIANCE
( SLOW SOLAR WIND, GALILEO)
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DENSITY TURBULENCE OUTER SCALE
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DENSITY TURBULENCE OUTER SCALE

• Radial dependence approximation
• L0( R ) A ( R / RS )m with
• A 0.24 RS and m 0.8 ,
• very close to linear.

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RESULTS

• A change of the turbulence regime occurs at the
  transition from the acceleration region to the
  region of the developed solar wind. (Also, Woo
  Armstrong, 1979)
• FR fluctuations measurements in the acceleration
  regions shows that flat flicker type spectra with
  p3 are also typical for magnetic field
  fluctuations (Chashei, Efimov, Bird et al.,
  2000).
• Recently (Chashei, Shishov Altyntsev) the
  evidences were found from the analysis of angular
  structure of the sources of microwave subsecond
  pulses for such spectra in the lower corona.
• The heliocentric distance of this change of
  turbulence regime is greater for the fast solar
  wind than for the slow solar wind during the
  period of low solar activity.

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RESULTS

• The fractional density fluctuations tend to
  increase slowly with increasing heliocentric
  distance.
• Turbulence outer scale increases approximately
  linear with increase of heliocentric distance in
  the range 10RS lt R lt 80 RS .
• Galileo data (1994-2002) no changes of slow wind
  turbulence during the solar activity cycle.

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TURBULENCE MODEL(acceleration region)

• The source of turbulence is a spectrum of Alfvén
  waves (magnetic field fluctuations), propagating
  away from the Sun.
• Slow and fast magnetosonic waves are generated
  locally via nonlinear interactions with Alfvén
  waves. Density fluctuations are dominated by slow
  magnetosonic waves.
• Turbulence is weak in the solar wind acceleration
  region (R lt 20 R?).
• The fractional level of turbulent energy
  increases with increasing heliocentric distance.
• Temporal power spectra are flat (? 0, ? ?
  3.0).
• No cascading of turbulence energy from the
  turbulence outer scale
• to smaller scales.

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TURBULENCE MODEL (change in
turbulence regime)

• The turbulence power spectrum of the developed
  solar wind in the inertial spectral range is
  defined by nonlinear cascading processes.
• Source of turbulence energy l.f. (outer
  scale) Alfven waves.
• Nonlinear generation of magnetosonic waves
  (density fluctuations) (Spangler Spitler, Ph.
  Pl., 2004).
• Spectra
• Kolmogorov (p11/3) or
• Iroshnikov-Kraichnan (p7/2) spectra.
• The change in turbulence regime is caused by the
  increase of fractional turbulence level (and
  increase of fractional level of fast magnetosonic
  waves compared with slow magnetosonic waves).
• The more distant transition for the fast solar
  wind may be explained by the lower value of the
  plasma parameter ? 4?P/B2 , i.?. by stronger
  ambient magnetic fields above the coronal holes.

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TURBULENCE MODEL (outer scale)

• Data are related to the region of formed solar
  wind flow.
• Model WkC1k-n at kltk0 , WkC2 k-m at kgtk0
  linear (WKB) propagation of Alfven waves at kltk0
  nonlinear cascading at kgtk0 (Kolmogorov,
  Kraichnan, 4-waves interactions) equal linear
  and nonlinear increments at kk0 k0 (R, n, m).
  LF spectrum can be assumed as flicker spectrum
  with n1 (Helios gtDenscat, Beinroth Neubauer,
  1983 Ulysses gt Hourbury Balogh, 2001).
• Comparison of the models with observational data
  best agreement at n1 is found for the Kraichnan
  turbulence.

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CONCLUSIONS

• Turbulence regimes in the acceleration region and
  in the formed solar wind are strongly different.
• Sufficiently good agreement between the
  observational data and the model.