Maps of z–v_z of the particles in the ‘solar neighbourhood’. The panels are colour-coded by number density (left column), density contrast (middle left column), radial velocity (middle right column), and azimuthal velocity (right column). The maps in the first row are at t = 0 Gyr. Second, third, and fourth rows correspond to 0.6 Gyr after the first pericentre passage, 0.5 Gyr after the second pericentre passage, and 0.5 Gyr after the third pericentre passage, respectively. Prominent one-arm phase spirals emerge in the solar neighbourhood after the first pericentre passage, becoming fainter after subsequent pericentre passages which have weaker impacts on the galactic disc.