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.