Ion acoustic wave
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An ion acoustic wave is a longitudinal oscillation of the ions (and the electrons) much like acoustic waves traveling in neutral gas. However, because the waves propagate through positively charged ions, ion acoustic waves can interact with their electromagnetic fields, as well as simple collisions. Ion acoustic waves can occur in an unmagnetized plasma or in a magnetized plasma parallel to the magnetic field. For a single species plasma, the waves are dispersionless with a speed (in the long wavelength limit) given by
where KB is Boltzmann's constant, M is the mass of the ion, Z is its charge, Te is the temperature of the electrons and Ti is the temperature of the ions. Normally γe is taken to be unity, on the grounds that the thermal conductivity of electrons is large enough to keep them isothermal on the time scale of ion acoustic waves, and γi is taken to be 3, corresponding to one-dimensional motion. In many plasmas, the electrons are much hotter than the ions, in which case the second term in the numerator can be ignored.
