Optical depths calculated in the differential study for (a) the Lyman alpha line; (b) the Hα line; and (c) the Pβ line 5 s afterthe onset of the beam, plotted against column depth (cm⁻²). Each subfigure shows the optical depth in the central wavelength of the line profile calculated for the thermal response (black line)and for models including excitation and ionisation rates from electron beams with fluxes in the range 10⁹ − 10¹² erg cm⁻² s⁻¹.