Ion acoustic wave

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An ion acoustic wave is a longitudinal oscillation of the ions (and the electrons) in an unmagnetized plasma or in a magnetized plasma parallel to the magnetic field. The waves are dispersionless with a speed given by

$v_s = \sqrt{\frac{\gamma_eZKT_e+\gamma_iKT_i}{M}}$

Normally γ_e 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 γ_i 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.