Synthetic spectral time series obtained from the 3D simulations. The top panel displays the spectra from the density I scenario, while the bottom panel shows the spectra from the same snapshot but using a density and pressure by a factor of 2.0 higher (density II). Purple vertical lines represent the stellar rest-frame wavelength of the metastable helium triplet in vacuum, and red tilted lines denote the planetary rest frame. Horizontal dotted lines indicate the transit contact points. The gray color map illustrates the transmission of the spectra, consistently following the planetary rest frame during the transit in both scenarios. In the higher density case (bottom), the contribution of the secondary red-shifted component becomes visible even during pre-transit phases.