Overview of the studied filament and Hα spectral profiles. Panel (a): Solar disk in the Hα line (6562.8 Å) captured by the Solar Dynamics Doppler Imager (SDDI) shortly before the filament eruption. The white box encloses the filament and outlines the field of view (FOV) of the images presented in Fig. 2. The small red box (50″ × 50″) surrounds a quiet-sun region from which the spectral profile is obtained and plotted in panel (b) by the red curve. The slit S1 is positioned in the direction of the leading part of the eruption (filament apex), whereas the slit S2 is used to track the lateral part of the eruption. Panel (b): Atlas solar spectrum of the Hα line (gray normalized to the continuum level) and convolution of the Atlas spectrum with the SDDI filter transmission profile (black). The quiet-sun SDDI spectral profile (see panel a) at μ ≈ 0.78 is also shown (red dots with spline interpolation), and an example of a strongly blueshifted profile (blue dots with spline interpolation) from the lateral part of the filament eruption at 05:44:50 UT (see Fig. 2). In the SDDI spectral profiles each dot represents the wavelength offset λ_s from the line center λ₀ (6562.8 Å) at which SDDI performs regular observations.