What Astronomers Want to Know about ISM Turbulence A. Lazarian (University of Wisconsin-Madison) Problem • Turbulence is essential to understand star formation, cosmic ray propagation etc. • High ISM Reynolds numbers (Re≡Lv/ν Rm≡Lv/ν m ) make brute force numerical testing impossible. Pragmatic Approach Essential astrophysical questions may be answered if one knows: •Turbulent spectrum: distribution of energy at different scales •Turbulent intermittency: properties of small volume with extreme conditions • Interaction of turbulence with cosmic rays Abstract While magnetized turbulence is an extremely complex phenomenon, a lot of advances in understanding astrophysical phenomena can be obtained if rather simple statistical measures are known. We show examples of the practical use of the measures of turbulence spectra and intermittency and discuss how the interactions with cosmic rays modify the spectrum. We describe techniques for obtaining the spectra of ISM velocity fluctuations and how they can be used to test both theory and numerics against observations. Summary • If turbulent spectra and turbulent anisotropies are known it is possible to quantify most of the essential ISM transport processes, e.g. heat transfer, CRs propagation. • Turbulence in ISM exhibits intermittency, I.e. small regions with extreme values of energy dissipation. This dissipation is important, but not sufficiently strong to radically change ISM chemistry. • Various ISM physical processes, e.g. CR instabilities, modify the spectrum of ISM turbulence. Thus the studies of turbulence spectra are very informative. • Turbulence spectra can be obtained from observations with new techniques, I.e. VCA and VCS. Both techniques have been successfully tested with synthetic and observational data. Acknowledgement: NSF grant AST 0307869 and NSF Center for Magnetic Self-Organization in Astrophysical and Laboratory Plasmas (CMSO). Questions ISM turbulence is complex. Is there any hope for theory advances? Is there any use of the theory? Turbulent Spectra Examples of Applying the Theory Spectrum Thermal conductivity Beresnyak & Lazarian 06 Turbulence spectrum (and anisotropy) allows to predict thermal conductivity of magnetized plasmas (e.g. in ISM). Above the curve turbulent advection of heat dominates electron conductivity. Lazarian 06 Turbulent Intermittency Effect of Cosmic Rays Re=10 4 Re=40 observed structures depend on Re Testing Theory with Observations Intermittency measure M A and M s are Alfven and sonic Mach numbers Intermittent energy deposition In some fraction of volume the energy density is 10 4 higher than the mean value. However, the fraction is small (cf. Falgarone et al. 2006) Beresnyak & Lazarian 06 Kowal & Lazarian 06 Lazarian & Beresnyak 06 Gyroresonace instability transfers energy from CR mean free path (mfp) to CR gyroradius. The instability is fed by the energy of compressible motions and both modifies turbulence and decreases mpf. New component Velocity Channel Analysis (VCA) Velocity Coordinate Spectrum (VCS) Lazarian & Pogosyan 00 Application of VCS to Arecibo HI data Successfully applied to SMC, Galactic HI, CO emission data High latitude HI Lazarian & Pogosyan 04 Lazarian & Pogosyan 06 Chepurnov & Lazarian 06 Emission lines Absorption lines Chepurnov et al. 06 S(v) is an observed spectral line,γ depends on the v spectral index α and observation geometry P 1 (k v ) ≡ S(v) e −ikv v dv ∫ 2 ∝ k v −γ Model with T=10 2 K fits the data for different resolutions. Spectral index is steep (-3.9) VCS uses info along v-axis. It does not need good spatial resolution for emission line studies and gets spectra when absorption is measured along a few lines of sight