Interstellar Turbulence and the Plasma Environment of the Heliosphere
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Interstellar Turbulence and the Plasma Environment of the Heliosphere
Steven R. SpanglerUniversity of Iowa
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The Warm Ionized Medium (WIM): where do stellar structures end and turbulence begin?
The sky as imaged by the Wisconsin H Alpha Mapper (WHAM; Haffner et al 2003, ApJS 149, 405)
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The Warm Ionized Medium (WIM) of the Interstellar Medium
• Density= 0.08 cc• B field = 3-4 microG• T=8000k• VA=23.3 km/sec• Hydrogen ionization:
>90 %• Helium ionization:
50%-100% neutral
See Haffner et al 2009, Rev. Mod. Phys. 81, 969 for full description
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Philosophical statement on turbulence: the solar wind should serve as a model of turbulence everywhere
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Power spectra of magnetic field and velocity in the solar wind
Podesta and Borovsky 2010, Phys. Plasm. 17, 112905
Outer scale
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What are the recent developments in studies of interstellar turbulence?
• Evidence for a relatively small outer scale ( ~ 5 parsecs) for WIM turbulence
• Claims that in the solar wind the power spectra of magnetic and velocity fluctuations differ (3/2 vs. 5/3)(Obs: J. Podesta and colleagues; Theory: S. Boldyrev and colleagues)
• Progress in understanding the dissipation mechanisms of solar wind turbulence, and by extension, all astrophysical turbulence (G. Howes and colleagues)
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Faraday Rotation in the corona and elsewhere
Rotation measure
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Cosmic magnetic fields here means the solar corona as well as that of the ISM and elsewhere
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Faraday Rotation as a turbulence diagnostic
A difference in Rotation Measure between two closely-spaced lines of sight
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Faraday rotation as a probe of interstellar plasma turbulence
“suitable for observers”
The rotation measure structure function
Minter and Spangler 1996, ApJ 458, 194
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The rotation measure structure function and the properties of interstellar turbulence
“It showed our intentions were serious…”
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The observed rotation measure structure function
Minter and Spangler 1996, ApJ 458, 194
5/32/3
Outer scale = 3.6 parsecs
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Recent studies have obtained rotation measure structure functions from large parts of the sky. They are always flatter than 5/3
Haverkorn et al ApJ 680, 362, 2008Oppermann et al A&A, in press
The “flatness” of rotation measure structure functions is an important diagnostic of interstellar turbulence
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What about the plasma environment of the Heliosphere?
Plasma of the Local Clouds similar (in many respects) to the WIM
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How do we infer the presence of turbulence
in the Very Local Interstellar Medium?
(Redfield and Linsky, ApJ 613, 1004, 2004)
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Spectra can measure central velocity, column density, and line width of each line isolated
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Physical properties of small clouds
• Ion density about 0.1/cc• Neutral fraction about 50%• Temperatures ~ 6700K• Clouds seem to be flowing from direction of
Scorpius-Centaurus Association
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Inferring cloud turbulence properties from high-resolution spectroscopy
Velocity centroid
Line width
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Line width due to Doppler motion of atoms or ions (thermal + turbulent)
With measurements of several atoms or ions (different m), can solve for T and \xi
Note: both T and \xi are line-of-sight values (Doppler effect)
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Capella
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Measurement of several lines leads to rms turbulent velocity
Redfield and Linsky 2004, ApJ 613, 1004
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Is the outer scale in the VLISM also small?
• Apparently not (?) Frisch et al (2010, ApJ 724, 1473) report relatively uniform B field over spatial extent of ~80 parsecs
• Direction of uniform field agrees with axis of IBEX “ribbon”, and heliospheric models
• Could still have turbulence with outer scale of 3-4 parsecs if amplitude is small.
• But, direction of Frisch et al (2010) field is at large angle with respect to galactic plane, like turbulent fluctuation.
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Are VLISM observations consistent with MHD turbulence possessing a pronounced “residual energy spectrum”?
Assume b and v spectra with residual energy spectrum
Assume at inner scale, fluctuations are Alfvenic
Then on large scales, fluctuations given by
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VLISM turbulence and residual energy spectrum
We know these parameters
Spangler, Savage, Redfield (ApJ 742, 30, 2011)
Would seem difficult to reconcile with uniform B over 80 parsecs
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A new age of opportunity for cosmic Faraday rotation measurements; the availability of the Karl G. Jansky
Very Large Array
• Lower noise receivers• Larger bandwidth• Continuous frequency
coverage
Thanks