Panel a: simulated Hα profiles showing red wing enhancement for the F10 (red line), F11 (green line), and F12 (blue line) models. Normalised intensity enhancement is on the y-axis, and wavelength relative to the Hα profile line centre on the x-axis. Panel b: excess of emission in the red wing of the profiles in panel “a” relative to the emission in the blue wing. Panel c: normalised Hα profiles from the M1 solar flare on May 24, 1987 reported in Wuelser & Marti (1989). The normalised Hα profile (green line), quiet Sun background level (grey line), and enhancement in emission (pink line) are shown for the A1 kernel at the time of the maximum of the HXR (15:24:55 UT). The y-axis shows normalised intensity and the wavelength relative to the Hα line central wavelength is shown on the x-axis. Panel d: excess emission in the red wing compared to the blue wing for the A1 kernel at 15:24:55 UT (green line). The red line shows a simulated fit of this excess based on a linear combination of contributions based on the F10 and F11 models.