Astronomy and Astrophysics – Astronomy
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.5001s&link_type=abstract
American Astronomical Society Meeting 210, #50.01; Bulletin of the American Astronomical Society, Vol. 39, p.162
Astronomy and Astrophysics
Astronomy
1
Scientific paper
Radioastronomical scintillation observations indicate a power law spectrum of plasma density fluctuations in the interstellar medium over a range of spatial wavenumbers of at least 7, and perhaps 11 orders of magnitude. This result is due to the work of Armstrong and colleagues since about 1980, and refers to plasma turbulence in the Diffuse Ionized Gas (DIG) phase of the interstellar medium. Jokipii has pointed out that the smooth variation of cosmic ray properties such as isotropy and confinement time with energy indicates that there must also be a power spectrum of magnetic fluctuations over the same wavenumber range. Presence of both magnetic field and plasma density fluctuations with the same scales suggests a similarity between interstellar turbulence and the extensively-measured turbulence in the solar wind. I will discuss what I consider major issues about the interpretation of these observations. (1) The interstellar spectrum should be truncated at a scale of 100-1000 a.u. due to ion-neutral collisions, but this does not seem to happen. (2) If interstellar turbulence generated even a small fraction of its energy density in fast magnetosonic waves, the heating rate due to fast mode damping would exceed the cooling capacity of the DIG. (3) Radio observations indicate that there is an outer scale of 3-4 parsecs in the DIG. This should effect cosmic rays with energies of 1015 - 1016 eV. (4) There is some observational evidence that interstellar medium turbulence is more compressive than solar wind turbulence. This is a clue to its nature and origin. (5) The relationship between plasma fluctuations such as electron density and magnetic field, and neutral gas characteristics such as HI density and velocity remains to be explored.
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