The Propagation Distance and Sources of Interstellar Turbulence

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The Propagation Distance and Sources of
Interstellar Turbulence
Steven R. Spangler
University of Iowa
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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
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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
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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
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Calculation of Collisional Heating Rate
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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.
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Implications for ISM Heating
Modification of Figure from Minter and Spangler
(1997), using solar-wind-derived CB2 instead of
Faraday rotation value
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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

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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
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Possible Explanations

• Unknown, local sources of interstellar turbulence
• Wave Percolation through a lacunose ISM
• Colossal physical misunderstanding

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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

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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
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The Physics of Ion-Neutral Interactions II
charge exchange

• Exchange creates fast neutral and slow ion

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Both processes relevant for conditions in the DIG

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

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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

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The Diffuse Ionized Gas (DIG) of the Interstellar
Medium

• Density 0.08 cc
• B field 3 microG
• T8000k
• VA23.3 km/sec
• Helium ionization 50-100 neutral

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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

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Solar Wind Data

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

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Empirical Compressibility Relation
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Application to Interstellar Medium
New estimate similar, but slightly higher than
MS96