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

MHD Turbulence in the Solar Wind

Turbulent spectra can be measured directly in the solar wind

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

Magnetic power spectrum

Density spectrum

Outer scale

Inner scale

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

Radio scintillations and measurement of turbulence in the interstellar medium

Scintillations Big Power Law in Sky

• Armstrong, Cordes, Rickett 1981, Nature

• Armstrong, Rickett, Spangler 1995, ApJ

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

J.R. Jokipii…”There are two big power laws in the sky”

Cosmic rays as probes of interstellar turbulence

Smoothness of CR spectrum indicates broad spectrum of irregularities

• S. Swordy, Univ. of Chicago

• Must be magnetic irregularities to resonate with all these energies

Cosmic ray and scintillation data indicate interstellar turbulence with

magnetic field and density fluctuations over many decades in wavenumber

Major questions about “Big Power Law” (my choices)

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

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

?

Ion-neutral collisions and the turbulent cascade

• Cascade goes through

• Oishi and MacLow 2006, ApJ 638,281

• Cho, Lazarian, & Vishniac 2002, ApJ 566,L40

#3- What is significance of “pulsar arcs”

Discovery by Stinebring et al (2001), Description & interpretation by Walker et al

(2004), Cordes et al (2006)

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

#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.

Summary

• 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

GPAP

Thanks