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

This study explores the existence and significance of a large inertial subrange in the interstellar medium (ISM), focusing on MHD turbulence and density fluctuations. It discusses the measurement of turbulent spectra in the solar wind, refractive scintillations, cosmic rays as probes of interstellar turbulence, ion-neutral collisions and the turbulent cascade, pulsar arcs, and the purging of MHD fast mode waves 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

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  1. 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

  2. MHD Turbulence in the Solar Wind

  3. Turbulent spectra can be measured directly in the solar wind Outer scale Magnetic power spectrum Inner scale R. Bruno, B. Bavassano, Ad Sp. Res. 35, 939, 2005 Density spectrum

  4. The Diffuse Ionized Gas (DIG) of the Interstellar Medium • Density= 0.08 cc • B field = 3 microG • T=8000k • VA=23.3 km/sec • Helium ionization: 50%-100% neutral

  5. Radio scintillations and measurement of turbulence in the interstellar medium

  6. Scintillations Big Power Law in Sky • Armstrong, Cordes, Rickett 1981, Nature • Armstrong, Rickett, Spangler 1995, ApJ

  7. 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

  8. J.R. Jokipii…”There are two big power laws in the sky” Cosmic rays as probes of interstellar turbulence

  9. Smoothness of CR spectrum indicates broad spectrum of irregularities • S. Swordy, Univ. of Chicago • Must be magnetic irregularities to resonate with all these energies

  10. Cosmic ray and scintillation data indicate interstellar turbulence with magnetic field and density fluctuations over many decades in wavenumber

  11. Major questions about “Big Power Law” (my choices) #1: Is a cascade occurring? Could be determined with measurement of bispectrum

  12. #2- Do ion-neutral collisions form a “stop band” in the interstellar spectrum? ?

  13. Ion-neutral collisions and the turbulent cascade • Cascade goes through • Oishi and MacLow 2006, ApJ 638,281 • Cho, Lazarian, & Vishniac 2002, ApJ 566,L40

  14. #3- What is significance of “pulsar arcs” Discovery by Stinebring et al (2001), Description & interpretation by Walker et al (2004), Cordes et al (2006)

  15. 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 doesn’t reproduce the observations • Stay tuned

  16. #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.

  17. 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

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