= Electrostatic plasma wave
{title2=$\delta\mathbf E=-i\mathbf k\phi$}
A <plasma> <wave> dominated by a longitudinal <electric field>. The leading <dispersion relation> is $\mathbf k\cdot\boldsymbol\epsilon\cdot\mathbf k=0$. In a <cold magnetized plasma>, this becomes $k_\perp^2S+k_\parallel^2P=0$. An exactly longitudinal finite-<wavenumber> solution must also satisfy the full <Maxwell equations>; the oblique electrostatic condition is usually an asymptotic large-<refractive index> approximation.
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