Progress in Radio Scattering Measurements of

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Progress in Radio Scattering Measurements of
Astrophysical Plasmas
Bill Coles, University of California, San Diego

• There has been significant progress in three
  areas
• Polar solar wind - small scale turbulence now
  appears to be dominated by oblique Alfvén waves.
• Warm interstellar plasma - turbulence is often
  anisotropic and may be intermittent
• Pulsar magnetosphere - first observations are now
  available to constrain theory!

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Plan view of an ecliptic observation
drifting intensity pattern
drifting phase pattern
incident plane wave
receiving antennas
Solar Wind
?
angular spectrum of plane waves
compact radio source
baseline
Sun
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Measurements of the Solar Wind

• Three distinct signatures of obliquely
  propagating Alfvén waves (ion cyclotron waves)
  are observed
• The density micro-structure is highly field
  aligned. The anisotropy decreases with increasing
  solar distance.
• The density spectrum is Kolmogorov at low
  frequencies, flattens at high frequencies, and
  cuts off near the ion inertial scale.
• The apparent velocity does not show any
  acceleration but near the Sun shows a large range
  of parallel velocities.

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Anisotropy vs Solar Distance
The vertical bars indicate variation not
statistical error
Model AR(R) of plasma
expected AR(R) for radio wave
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Spectra measured in the fast polar wind
Ulysses
Anisotropic Range
Paetzold Bird
VLA perp
VLBA
?ci/VA
Harmon and Coles
Grall et al., VLA par
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Measured IPS Parallel Velocity Distribution
upper envelope VFLOW VA
flow speed
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Wave Model

• In a damped WKB model electron Landau damping
  dominates and cuts the spectrum off at larger
  scales than observed.
• We added a crude model of a turbulent cascade in
  which
• the cascade balances the damping and
• the cascade is radial in wavenumber space.
• This pushed the spectrum out to smaller scales
  where it was cutoff by proton cyclotron damping.
• The 2-D cascade model spectra match observations
  well.

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Theoretical vs measured structure functions of
density fluctuations
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Upper bound on Solar wind flow speed
The group velocity of the waves is the Alfvén
speed, so the apparent velocity of the radio
diffraction pattern is the sum of the flow speed
and the Alfvén speed. The lower envelope of the
parellel velocity distribution is an upper bound
on the flow speed.
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Conclusions
Radio observations only measure density which
does not provide the energy, but having
identified a wave mode we can calculate the
dissipation.
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The Warm Interstellar Medium (WIM)
Composite map of Ha emission from WIM
(Finkbeiner 2004)
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Density spectrum of WIM by a variety of methods.
Armstrong, Rickett Spangler, 1995
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Intermittency
The level of turbulence varies by 104 even
between nearby objects. This suggests
intermittency, but could simply be caused by
different structures on different lines of
sight. New observations show that scattering
from a single compact region is very common on
paths less that a few kpc. Many of these paths
have no obvious structure, suggesting that the
turbulence is intermittent, but deeper
comparisons are necessary.
Anisotropy 2-D array measurements have been made
on a few highly scattered pulsars. These have
shown almost isotropic structure. Recent
observations of more common objects have shown AR
gt 41 so anisotropy may be more common than
realized.
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The phenomenon was discovered in this dynamic
spectrum of inter-stellar scintillation of psr
083406 by Stinebring et al.
A 2-D spectral analysis of this dynamic spectrum
(on log intensity scale with 60dB range) shows a
parabolic arc.
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Intra-day variability in the quasar J18193845
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The Double Pulsar
Tp 3 s
Tp 23 ms
pulsar wind
to Earth
A
B
magnetosphere
Ref Lyutikov astro-ph/0502503
The short period pulsar (A) has a much stronger
wind. Both pulsars scintillate in the
interstellar plasma, and their scintillations can
be separated because their periods are different.
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The cross-correlation of the A and B
scintillations
We can measure the apparent separation of A and B
near the eclipse very accurately. A will appear
closer to B because of refraction in the
magnetosphere of B. We can also measure the
apparent separation using the Shapiro delay of
the A pulse as it travels through the
gravitational potential of B.