  The physics of plasmas containing positive and negative ions is discussed
with special attention to the recently produced pair-ion plasma containing ions
of equal mass and opposite charge. The effects of the density gradient in the
direction perpendicular to the ambient magnetic field vector, observed in the
experiment, are discussed. In addition, the possible presence of electrons is
discussed in the context of plasma modes propagating at an angle with respect
to the magnetic field vector. The electrons may either be added to the plasma
or enter the plasma attached to negative ions and then become released from the
ions as a result of collisions. It is shown that the electron plasma mode in a
cold plasma may become a backward mode in the presence of a density gradient,
and this behavior may be controlled either by the electron number density or
the mode number in the perpendicular direction. In plasmas with hot electrons
an instability may develop, driven by the combination of electron collisions
and the density gradient, and in the regime when the ions' response is similar
to a sound mode, i.e., for un-magnetized ions, implying mode frequencies much
above the ion gyro-frequency or mode wave-lengths shorter than the ion
gyro-radius. In the case of a pure pair-ion plasma, for lower frequencies and
for parameters close to those used in the recent experiments, the perturbed
ions may feel the effects of the magnetic field. In this case the plasma mode
also becomes backward, resembling features of an experimentally observed but
yet unexplained backward mode.
