2D cuts at ϕ = 0° of the evolution of selected quantities in Hβ at t = 2900 s, t = 5400 s, and t = 12 500 s: mass density variations, ρ − ρ_rel (top row panels), velocity variations v − v_rel (middle row panels), and ϕ velocity component, v_ϕ (bottom row panels), perpendicular to the plane of the cut with blue (respectively red) colour scales indicating observer-directed (respectively oppositely directed) velocities. An animation of this figure is available online.