Electron density power spectrum in the local interstellar medium
J. W. Armstrong, B. J. Rickett, S. R. Spangler;
ApJ, 1995, 443, 209
ABSTRACT:Interstellar scintillation (ISS), fluctuations in the amplitude and phase of radio waves caused by scattering in the interstellar medium, is
important as a diagnostic of interstellar plasma turbulence.
ISS is also of
interest because it is noise for other radio astronomical observations.
The
unifying concern is the power spectrum of the interstellar electron
density.
Here we use ISS observations through the nearby (less than or approximately
=1 kpc) (ISM) to estimate the spectrum.
From measurements of angular
broadening of pulsars and extragalactic sources, decorrelation bandwidth of
pulsars, refractive steering of features in pulsar dynamic spectra,
dispersion measured fluctuations of pulsars, and refractive scintillation
index measurements, we construct a composite structure function that is
approximately power law over 2 x 106 m less than scale less than
1013 m.
The data are consistent with the structure function having a
logarithmic slope versus baseline less than 2; thus there is a meaningful
connection between scales in the radiowave fluctuation field and the scales in the
electron density field causing the scattering.
The data give an upper limit to
the inner scale, lo less than or approximately
108 m and are consistent with much smaller values.
We construct a
composite electron density spectrum that is approximately power law over at
least the approximately = 5 decade wavenumber range 10-13/m
less than wavenumber less than 10-8/m and that may extend to
higher wavenumbers.
The average spectral index of electron density over
this wavenumber range is approximately = 3.7, very close to the value
expected for a Kolmogorov process.
The outer scale size, Lo, must
be greater than or approximately = 1013 m (determined from
dispersion measure fluctuations).
When the ISS data are combined with
measurements of differential Faraday rotation angle, and gradients in the average
electron density, constraints can be put on the spectrum at much smaller wave
numbers.
The composite spectrum is consistent with a Kolmogorov-like power law
over a huge range (10 or more decades) of spatial wavenumber with an infrared
outer scale Lo greater than or approximately
1018m.
This power-law subrange-expressed as ratio of outer to inner scales-is
comparable to or larger than that of other naturally occurring turbulent fluids,
such as the oceans or the solar wind.
We outline some of the theories for
generating and maintaining such a spectrum over this huge wavenumber range.
KEYWORDS: electron density profiles, interstellar matter, power spectra, scintillation, astronomical models, plasma turbulence, pulsars, radio astronomy
PERSOKEY:turbulence, ,
CODE: armstrong95