Interstellar Turbulence and the Plasma Environment of the Heliosphere

Steven R. SpanglerUniversity of Iowa

The Warm Ionized Medium (WIM): where do stellar structures end and turbulence begin?

The sky as imaged by the Wisconsin H Alpha Mapper (WHAM; Haffner et al 2003, ApJS 149, 405)

The Warm Ionized Medium (WIM) of the Interstellar Medium

• Density= 0.08 cc• B field = 3-4 microG• T=8000k• VA=23.3 km/sec• Hydrogen ionization:

>90 %• Helium ionization:

50%-100% neutral

See Haffner et al 2009, Rev. Mod. Phys. 81, 969 for full description

Philosophical statement on turbulence: the solar wind should serve as a model of turbulence everywhere

Power spectra of magnetic field and velocity in the solar wind

Podesta and Borovsky 2010, Phys. Plasm. 17, 112905

Outer scale

What are the recent developments in studies of interstellar turbulence?

• Evidence for a relatively small outer scale ( ~ 5 parsecs) for WIM turbulence

• Claims that in the solar wind the power spectra of magnetic and velocity fluctuations differ (3/2 vs. 5/3)(Obs: J. Podesta and colleagues; Theory: S. Boldyrev and colleagues)

• Progress in understanding the dissipation mechanisms of solar wind turbulence, and by extension, all astrophysical turbulence (G. Howes and colleagues)

Faraday Rotation in the corona and elsewhere

Rotation measure

Cosmic magnetic fields here means the solar corona as well as that of the ISM and elsewhere

Faraday Rotation as a turbulence diagnostic

A difference in Rotation Measure between two closely-spaced lines of sight

Faraday rotation as a probe of interstellar plasma turbulence

“suitable for observers”

The rotation measure structure function

Minter and Spangler 1996, ApJ 458, 194

The rotation measure structure function and the properties of interstellar turbulence

“It showed our intentions were serious…”

The observed rotation measure structure function

Minter and Spangler 1996, ApJ 458, 194

5/32/3

Outer scale = 3.6 parsecs

Recent studies have obtained rotation measure structure functions from large parts of the sky. They are always flatter than 5/3

Haverkorn et al ApJ 680, 362, 2008Oppermann et al A&A, in press

The “flatness” of rotation measure structure functions is an important diagnostic of interstellar turbulence

What about the plasma environment of the Heliosphere?

Plasma of the Local Clouds similar (in many respects) to the WIM

How do we infer the presence of turbulence

in the Very Local Interstellar Medium?

(Redfield and Linsky, ApJ 613, 1004, 2004)

Spectra can measure central velocity, column density, and line width of each line isolated

Physical properties of small clouds

• Ion density about 0.1/cc• Neutral fraction about 50%• Temperatures ~ 6700K• Clouds seem to be flowing from direction of

Scorpius-Centaurus Association

Inferring cloud turbulence properties from high-resolution spectroscopy

Velocity centroid

Line width

Line width due to Doppler motion of atoms or ions (thermal + turbulent)

With measurements of several atoms or ions (different m), can solve for T and \xi

Note: both T and \xi are line-of-sight values (Doppler effect)

Capella

Measurement of several lines leads to rms turbulent velocity

Redfield and Linsky 2004, ApJ 613, 1004

Is the outer scale in the VLISM also small?

• Apparently not (?) Frisch et al (2010, ApJ 724, 1473) report relatively uniform B field over spatial extent of ~80 parsecs

• Direction of uniform field agrees with axis of IBEX “ribbon”, and heliospheric models

• Could still have turbulence with outer scale of 3-4 parsecs if amplitude is small.

• But, direction of Frisch et al (2010) field is at large angle with respect to galactic plane, like turbulent fluctuation.

Are VLISM observations consistent with MHD turbulence possessing a pronounced “residual energy spectrum”?

Assume b and v spectra with residual energy spectrum

Assume at inner scale, fluctuations are Alfvenic

Then on large scales, fluctuations given by

VLISM turbulence and residual energy spectrum

We know these parameters

Spangler, Savage, Redfield (ApJ 742, 30, 2011)

Would seem difficult to reconcile with uniform B over 80 parsecs

A new age of opportunity for cosmic Faraday rotation measurements; the availability of the Karl G. Jansky

Very Large Array

• Lower noise receivers• Larger bandwidth• Continuous frequency

coverage

Thanks