Weak spin-forbidden Paschen-Back line ($I_{\mathrm{weak}}\propto (\beta /{\mu }_{B}B{)}^2$)

At large but finite magnetic field, opposite-spin admixtures in approximate product eigenstates let nominally spin-forbidden electric-dipole channels borrow intensity. Their amplitudes scale as spin-orbit interaction energy divided by Zeeman energy; their intensities therefore vanish quadratically in that ratio.