Magnetic Fields in Diffuse Media by Alexander Lazarian Elisabete M. de Gouveia Dal Pino & Claudio Melioli

Author:Alexander Lazarian, Elisabete M. de Gouveia Dal Pino & Claudio Melioli
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

12.2.2 Turbulence in Astrophysical Fluids

Neither of these models take into account turbulence, which is ubiquitous in astrophysical environments. Indeed, plasma flows at high Reynolds numbers are generically turbulent, since laminar flows are then prey to numerous linear and finite-amplitude instabilities. This is sometimes driven turbulence due to an external energy source, such as supernova in the ISM (Norman and Ferrara 1996; Ferrière 2001), merger events and AGN outflows in the intercluster medium (ICM) (Subramanian et al. 2006; Enßlin and Vogt 2006; Chandran 2005), and baroclinic forcing behind shock waves in interstellar clouds. In other cases, the turbulence is spontaneous, with available energy released by a rich array of instabilities, such as the MRI in accretion disks (Balbus and Hawley 1998), the kink instability of twisted flux tubes in the solar corona (Galsgaard and Nordlund 1997; Gerrard and Hood 2003), etc. Whatever its origin, observational signatures of astrophysical turbulence are seen throughout the universe. The turbulent cascade of energy leads to long “inertial ranges” with power-law spectra that are widely observed, e.g. in the solar wind (Leamon et al. 1998; Bale et al. 2005), and in the ICM Schuecker et al. (2004); Vogt and Enßlin (2005).

Figure 12.3 illustrates the so-called “Big Power Law in the Sky” of the electron density fluctuations. The original version of the law was presented by Armstrong et al. Armstrong et al. (1995) for electron scattering and scintillation data. It was later extended by Chepurnov et al. (Chepurnov and Lazarian 2010) who used Wisconsin Hα Mapper (WHAM) electron density data. We clearly see the power law extending over many orders of spatial scales and suggesting the existence of turbulence in the interstellar medium. With more surveys, with more developed techniques we are getting more evidence of the turbulent nature of astrophysical fluids. For instance, for many years non-thermal line Doppler broadening of the spectral lines was used as an evidence of turbulence.2 The development of new techniques, namely, Velocity Channel Analysis (VCA) and Velocity Correlation Spectrum (VCS) in a series of papers by Lazarian and Pogosyan (2000, 2004, 2006, 2008) enabled researchers to use HI and CO spectral lines to obtain the power spectra of turbulent velocities (see Lazarian (2009) for a review and references therein).

Fig. 12.3Turbulence in the interstellar gas of the Milky Way as revealed by electron density fluctuations. “Big Power Law in the Sky” Armstrong et al. (1995) extended using WHAM data. The slope corresponds to that of Kolmogorov turbulence. From Chepurnov and Lazarian (2010)

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