The Propagation Distance and Sources of Interstellar Turbulence

1. The Propagation Distance and Sources of Interstellar Turbulence Steven R. Spangler University of Iowa

2. Entities responsible for Tiny Scale Structures and Extreme Scattering Events are probably part of interstellar plasma turbulence---”solitary waves” or “coherent structures”. The Diffuse Ionized Gas (DIG) is a partially ionized plasma

3. “Graduate school courses in plasma physics are usually restricted to the discussion of fully ionized plasmas. Practitioners of the subject, however, are aware of the fact that media of research interest are not generally fully ionized. Laboratory physicists know that neutral gas is present and is a nuisance, in that it can collisionally damp phenomena of interest. Astronomers have a Manichean view of ionization in the universe; they assume that their media are either completely neutral and obedient to the laws of hydrodynamics, … or completely ionized and describable by single fluid magnetohydrodynamics.” SRS, Physics of Plasmas 10, 2169, 2003

4. An apparently important process in the thermodynamics of the the DIG: Ion-Neutral Collisional Heating Spangler 1991, ApJ 376,540; Minter and Spangler 1997, ApJ 485, 182

5. Calculation of Collisional Heating Rate

6. Calculation assumed spectrum of turbulence as given, then calculated the heating rate as the turbulence decays. No attempt to give self-consistent description of turbulence.

7. Implications for ISM Heating Modification of Figure from Minter and Spangler (1997), using solar-wind-derived CB2 instead of Faraday rotation value

8. A Gratifying Result • Calculation suggests turbulent heating might provide important process in the thermodynamics of the interstellar medium. • Further discussion and support from data in Minter and Balser, ApJ 484, L133, 1997

9. An odd and undesired consequence of these ideas: the propagation distance of interstellar turbulence • Damping rate on neutral helium S-1 Propagation distance of turbulence Vastly smaller than typical distance to hypothesized sources of turbulence

10. Possible Explanations • Unknown, local sources of interstellar turbulence • “Wave Percolation” through a lacunose ISM • Colossal physical misunderstanding

11. The microphysics of ion-neutral damping • Mechanism 1: induced dipole moment in neutral by ion • Mechanism 2: charge exchange • Astronomers can benefit from interest in plasma physics; ion-neutral interactions are important in Tokamak confinement

12. The Physics of Ion-Neutral Interactions I • Interactions due to induced dipole moment of neutral atoms • Collision frequency: Ion-neutral cross section: Cross sections cm2

13. The Physics of Ion-Neutral Interactions II: charge exchange Exchange creates fast neutral and slow ion

14. Both processes relevant for conditions in the DIG It is probably worthwhile to revisit the microphysics of MHD wave damping via ion-neutral interactions

15. Future Research Directions • Detailed study of damping of MHD waves in a partially ionized plasma, paying attention to energy flow • Laboratory experiments to test those results • Astronomical observational tests for anomalous neutral heating in the DIG and similar ISM phases

16. 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

17. Estimates of PB(k) • Radio scintillations measurements sensitive only to density n, not B or V • Minter and Spangler (1996 ApJ 458, 184) used Faraday rotation to retrieve CB2 • Approach here: use slow solar wind as a model plasma to determine n-B relation;

18. Solar Wind Data • Used Wind spacecraft data from NSSDC • Analysed 50 intervals of one hour duration. • All in slow solar wind (V < 400 km/sec) • Parameters calculated:

19. Empirical Compressibility Relation

20. Application to Interstellar Medium New estimate similar, but slightly higher than MS96