= Rotation measure
{title2=$\mathrm{RM}=a_0\int n_e B_\parallel\,dz$}
For the high-frequency <Faraday effect> in a cold <plasma>, the rotation measure is the coefficient in $\Delta\phi=\lambda^2\mathrm{RM}$. It is a density-weighted line integral of the line-of-sight <magnetic field>; the coefficient $a_0$ depends on the units. A sign convention for the line of sight fixes its sign. Correlations across an extended source can constrain a foreground <magnetic energy spectrum>, subject to statistical assumptions.
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