= Long-path projection of a magnetic correlation
{title2=$C_{\mathrm{RM}}(s)\simeq a_0^2n_e^2L\int H(k_\perp)e^{i\mathbf k_\perp\cdot\mathbf s}d^2k_\perp/(2\pi)^2$}
For constant <Electron> density, common path length $L$, and a <solenoidal isotropic spectral tensor>, the rotation-measure <correlation> contains the window $|\int_0^Le^{ik_zz}dz|^2$. As $L$ becomes large compared with the <correlation length>, this window acts as $2\pi L\delta(k_z)$ within the <integral>. The transverse projector then has $zz$ component one, yielding the displayed two-dimensional <Fourier transform>. For finite $L$, the windowed integral retains longitudinal modes and endpoint corrections. The method of using such <rotation measure> correlations to infer magnetic spectra is developed by https://arxiv.org/abs/astro-ph/0302426[Enßlin and Vogt].
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