= Entropy mode
{title2=$Ds=0$}
An <entropy mode> is a perturbation of <entropy> and <density> with no leading <pressure> or <velocity> perturbation in a uniform ideal fluid. For an <adiabatic equation of state> define $s=\delta p/p_0-\gamma\delta\rho/\rho_0$. An <entropy> perturbation can have $\delta p=0$ and $\delta\rho/\rho_0=-s/\gamma$, so it has zero <frequency> in the local fluid frame. In anisotropic <reduced magnetohydrodynamics>, $Ds=0$ with $D=\partial_t+\mathbf u_\perp\cdot\nabla_\perp$, and <turbulence> mixes its <variance> as a <passive scalar>.
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