The Propagation Distance and Sources of Interstellar Turbulence

Steven R. Spangler University of Iowa

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

“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

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

Calculation of Collisional Heating Rate

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.

Implications for ISM Heating

Modification of Figure from Minter and Spangler (1997), using solar-wind-derived CB

2 instead of Faraday rotation value

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

An odd and undesired consequence of these ideas: the propagation distance of interstellar

turbulence

• Damping rate on neutral helium 11 12

0 / 2 6.5 10 6.5 10n S-1

Propagation distance of turbulence6

1712

2.3 103 10

6.5 10AVl cm

Vastly smaller than typical distance to hypothesized sources of turbulence

Possible Explanations

• Unknown, local sources of interstellar turbulence

• “Wave Percolation” through a lacunose ISM

• Colossal physical misunderstanding

1 1v

fv f E v B f f

t c

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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

The Physics of Ion-Neutral Interactions I

• Interactions due to induced dipole moment of neutral atoms

• Collision frequency: 0 vn

Ion-neutral cross section:

22.2 ( )( )m

e M mv

v Mm

Cross sections 1510 cm2

The Physics of Ion-Neutral Interactions II: charge exchange

Exchange creates fast neutral and slow ion

Both processes relevant for conditions in the DIG

It is probably worthwhile to revisit the microphysics of MHD wave damping via ion-neutral interactions

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

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

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 CB

2

• Approach here: use slow solar wind as a model plasma to determine n-B relation;

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:

Empirical Compressibility Relation

Application to Interstellar Medium

New estimate similar, but slightly higher than MS96