Breathing signature around spiral arms. Upper panel: breathing signature in the isolated model. Colour map shows the breathing velocity, v_{z, > 0}−v_{z, > 0}. Contours indicate the normalised density, Σ(R, ϕ)/Σ₀(R). Black dots indicate the density peaks. Lower panel: same as the upper panel but in the perturbed model. In the perturbed model (tidally induced arms), the breathing velocity minimum is displaced from the density peak to the trailing side, unlike in the isolated model (dynamic arms) where they are aligned.