  Ion sound waves are studied in a plasma subject to gravitational field. Such
systems are interesting by exhibiting a wave growth that is a result of energy
flux conservation in inhomogeneous systems. The increasing wave amplitude gives
rise to an enhanced interaction between waves and plasma particles that can be
modeled by a modified Korteweg-de Vries equation. Analytical results are
compared with numerical Particle-in-Cell simulations of the problem. Our code
assumes isothermally Boltzmann distributed electrons while the ion component is
treated as a collection of individual particles interacting through collective
electric fields. Deviations from quasi neutrality are allowed for.
