= Critical balance
{title2=$\tau_{\mathrm{nl}}\sim\tau_{\mathrm{wave}}$}
<Critical balance> is a scale-dependent ordering in which the wave-propagation time and nonlinear interaction time are comparable. For strong <Alfvénic turbulence>, $k_\parallel v_A\sim k_\perp\delta u_\perp$. For <electron magnetohydrodynamics>, the linear rate is $v_Ad_i k_\parallel k_\perp$ and the nonlinear rate is $v_Ad_i k_\perp^2\delta B/B_0$, so <critical balance> instead gives $\delta B/B_0\sim k_\parallel/k_\perp$. This assumption is an estimate of decorrelation dynamics, not an exact equality at every point.
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