= Magnetic second-moment tensor
{title2=$M_{ij}=\langle B_iB_j\rangle$}
For a resolved <magnetic field>, the unaveraged tensor $B_iB_j$ describes <magnetic tension>; the isotropic <magnetic pressure> is treated separately in the <magnetohydrodynamic total pressure>. Its spatial or ensemble average need not have <matrix rank> one. <Statistical isotropy> in three dimensions gives $\langle B_iB_j\rangle=\langle B^2\rangle\delta_{ij}/3$, by taking the <trace>. In <Alfvén velocity> units this tensor has dimensions of squared <velocity>.
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