= Dimensional freedom of an MHD spectrum
{title2=$E(k)=\epsilon^{2/3}k^{-5/3}\Phi(v_A\epsilon^{-1/3}k^{1/3})$}
In a local isotropic hydrodynamic inertial-range model with only $\epsilon$ and $k$, dimensional analysis fixes the <Kolmogorov 1941 theory> spectrum. A mean <magnetic field> supplies the independent <Alfvén speed>, permitting the arbitrary dimensionless function in the displayed formula even before anisotropy adds parallel scales. Cascade-time assumptions such as weak decorrelated interactions or <critical balance> are therefore necessary to select an MHD spectrum.
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