= Weak Alfvénic cascade
{title2=$E_\perp(k_\perp)\sim(\epsilon v_A/l_\parallel)^{1/2}k_\perp^{-2}$}
For balanced anisotropic <Alfvénic turbulence>, suppose $\tau_A=l_\parallel/v_A\ll\tau_{\rm nl}=l_\perp/\delta z$ and resonant interactions preserve the parallel scale during the perpendicular cascade. Random encounter accumulation gives $\tau_{\rm cas}=v_Al_\perp^2/(\delta z^2l_\parallel)$. Constant <energy flux> then gives $\delta z\propto l_\perp^{1/2}$ and the displayed perpendicular <turbulent energy spectrum>. Unlike isotropic phenomenology, the fixed parallel scale is independent of the decreasing perpendicular scale.
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