[O III] emission lines (July 2019 FIES spectrum) and the corresponding electron density in velocity space. Shown are [O III]λ5007 (solid blue) and [O III]λ4363 (dotted green). Their units are in erg s⁻¹ cm⁻² Å⁻¹, and are indicated in blue on the right axis. The black solid line is the corresponding electron density, n_e, per resolution element (units in cm⁻³ on the left axis). The n_e is plotted only within the velocity range [−600,+600] km s⁻¹, beyond which the noise dominates.