Sketch of the geometrical model of the disturbance. The red circles depict the disturbance front location at different time steps. The blue circle represents the Sun and it is centred at the axis origin. We label the geometrical parameters we used in the model. The labels r_sh and R_sh are the distance of a given point, P, located at the disturbance from the solar centre (O) and the disturbance centre (C), and φ is the ∡ COP angle, i.e. the longitudinal separation angle from the disturbance source region.