  It is economical to devise ways and means to simplify a multi-species
particle system. The single fluid MHD description of a plasma is sufficiently
adequate to study plasma phenomena . A plasma containing multi-species of
particles can also be described by representing each particle species by the
corresponding fluid. A partially ionized plasma consisting of electrons, ions
and the neutral particles can be described as a single fluid by appropriately
combining the dynamics of each of the species. The three component plasma can
also be reduced to a two-fluid system. The most obvious way is to combine the
electron and the ion fluids to obtain a single fluid in the MHD limit and the
neutral particles to form the second fluid. The other two possibilities are 1)
to combine electron fluid and the neutral fluid into one fluid, here,
christened as ENe fluid, and treat the ions as the second fluid and 2) to
combine the ion fluid and the neutral fluid into one fluid, here, christened as
INe fluid, and treat the electrons as the second fluid. It is found that the
process of combining the electrons and the neutrals endows the neutrals with a
negative electric charge. So, here is a new plasma with heavy ( with mass of
the neutral particle assuming the electron mass to be negligible) negatively
charged particles along with the positively charged ions. One could in
principle, similarly treat the second possibility. However , since there can be
a huge variation between the relative masses of the neutrals and the ions, the
system turns out to be more complex and its discussion is deferred until later.
Here, after establishing the framework for the two-fluid ENe-Ion system, the
characteristic electrostatic wave mode of this novel two-fluid plasma is
determined. The wave frequency is found to be a function of the collision
frequencies in addition to the dependence on usual parameters.
