The Big Power Law in the Sky: Existence and Significance of a Large Inertial Subrange in the ISM

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The Big Power Law in the Sky Existence and
Significance of a Large Inertial Subrange in the
ISM
Steven R. Spangler, Department of Physics and
Astronomy
University of Iowa
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MHD Turbulence in the Solar Wind
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Turbulent spectra can be measured directly in the
solar wind
Outer scale
Magnetic power spectrum
Inner scale
Density spectrum

• R. Bruno, B. Bavassano, Ad Sp. Res. 35, 939, 2005

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The Diffuse Ionized Gas (DIG) of the Interstellar
Medium

• Density 0.08 cc
• B field 3 microG
• T8000k
• VA23.3 km/sec
• Helium ionization 50-100 neutral

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Radio scintillations and measurement of
turbulence in the interstellar medium
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Scintillations Big Power Law in Sky

• Armstrong, Cordes, Rickett 1981, Nature
• Armstrong, Rickett, Spangler 1995, ApJ

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Refractive scintillations (low frequency
variability) show related density fluctuations in
2 widely separated k-bands
Refractive scintiilations dominated by
irregularities with sizes of order
Refractive scintillation indices mainly
consistent with irregularities 5 decades smaller
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J.R. JokipiiThere are two big power laws in the
sky

• Cosmic rays as probes of interstellar turbulence

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Smoothness of CR spectrum indicates broad
spectrum of irregularities

• S. Swordy, Univ. of Chicago
• Must be magnetic irregularities to resonate with
  all these energies

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Cosmic ray and scintillation data indicate
interstellar turbulence with magnetic field and
density fluctuations over many decades in
wavenumber
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Major questions about Big Power Law (my choices)

• 1 Is a cascade occurring? Could be determined
  with measurement of bispectrum

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2- Do ion-neutral collisions form a stop band
in the interstellar spectrum?
?
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Ion-neutral collisions and the turbulent cascade

• Cascade goes through
• Oishi and MacLow 2006, ApJ 638,281
• Cho, Lazarian, Vishniac 2002, ApJ 566,L40

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3- What is significance of pulsar arcs

• Discovery by Stinebring et al (2001), Description
  interpretation by Walker et al (2004), Cordes
  et al (2006)

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Pulsar arcs and interstellar turbulence

• Latest word, Trang and Rickett, astro-ph/702210
• Turbulence producing scattering concentrated in a
  few (1?) screen
• Kolmogorov broadening function doesnt reproduce
  the observations
• Stay tuned

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4 What purges the ISM of MHD fast mode waves?

• Even a small fraction of interstellar turbulence
  in form of fast mode waves would overheat the
  ISM.

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Summary
GPAP

• The existence of density fluctuations on wide
  range of spatial scales is well established
• There are magnetic field fluctuations on the same
  scales
• This may indicate solar wind-like turbulence in
  the DIG
• Radio scintillation observations show many
  anomalies
• The existence and significance of mesoscales in
  the DIG is not yet established
• Further studies of solar wind turbulence, and
  fundamental plasma physics investigations are
  important components of progress

Thanks