Illustrative 1D numerical computations of the effect of Earth atmospheric seeing S (arcseconds) on the modeled signed degree of linear polarization P_s in the Hα filter (λ₀ = 656 nm). The atmosphere is assumed to be horizontally homogeneous except for the imposed gas-density variations. Increasing S progressively attenuates the wavelike modulation in P_s. These calculations are intended to illustrate the sensitivity of the signal to seeing, not to reproduce the exact atmospheric conditions during the ExPo observations.