Estimates of the filling fraction for ionized particles in the interstellar and intergalactic medium range from a few percent to 100 percent. As shown by Earth's ionosphere where the ionization fraction is only a few percent, plasma processes can be important even for low filling fraction. Plasmas are a combination of neutrals, ions, electrons and fields that have conductive and collective effects and where the interparticle dynamics is not dominated simply by binary collisions. This condition applies for most astrophysical systems. Even though space plasmas maintain quasi-neutrality to within less than about 1 part per million, there can still be substantial currents, convection, plasma flows, plasma waves and shocks and other plasma effects that interconnected regions over vast expanse as demonstrated by direct measurements of solar system space plasmas. Direct evidence of galactic magnetic fields are now available. Radio jets, interstellar shocks and many astrophysical phenomena now appear to involve important plasma effects. Although normally less dynamically important than the gravitational force, especially at cosmic scale, plasma astrophysics will likely be at the cutting edge of 21st century astrophysics and cosmology studies.

Please see Plasmas.org for more information.


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