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Invert the two-dimensional Fourier transform from part (c):In polar coordinates, the angular integral is $2\pi J_0(ks)$, where $J_0$ is a Bessel function of the first kind. The Fourier-Hankel normalization for an isotropic spectrum therefore yieldsThis Hankel inversion of a rotation-measure correlation recovers the scalar spectral tensor coefficient. To obtain the shell-integrated magnetic energy spectrum, take the trace of the three-dimensional tensor:With magnetic energy density $B^2/(8\pi )$ in Gaussian units, the one-dimensional convention ${\int }_0^{\infty }{E}_B(k)dk=\mathbb{E}|\mathbf{B}{|}^2/(8\pi )$ gives $\boxed{E_B(k)=k^{2}H(k)/(8{\pi }^3)}$. A spectrum normalized instead to $\mathbb{E}|\mathbf{B}{|}^2/2$ has coefficient $k^{2}H(k)/(2{\pi }^2)$; the physical normalization must be stated.