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We use the second data releases of the ESA Gaia astrometric survey and thehigh-resolution GALAH spectroscopic survey to analyse the structure of ourGalaxy's disc components. With GALAH, we separate the alpha-rich and alpha-poordiscs (with respect to Fe), which are superposed in both position and velocityspace, and examine their distributions in action space. We study thedistribution of stars in the zV_z phase plane, for both V_phi and V_R, andrecover the remarkable "phase spiral" discovered by Gaia. We identify theanticipated quadrupole signature in zV_z of a tilted velocity ellipsoid forstars above and below the Galactic plane. By connecting our work with earlierstudies, we show that the phase spiral is likely to extend well beyond thenarrow solar neighbourhood cylinder in which it was found. The phase spiral isa signature of corrugated waves that propagate through the disc, and theassociated non-equilibrium phase mixing. The radially asymmetric distributionof stars involved in the phase spiral reveals that the corrugation, which ismostly confined to the alpha-poor disc, grows in z-amplitude with increasingradius. We present new simulations of tidal disturbance of the Galactic disc bythe Sagittarius (Sgr) dwarf. The effect on the zV_z phase plane lasts >2 Gyrbut a subsequent disc crossing wipes out the coherent structure. We find thatthe phase spiral was excited < 0.5 Gyr ago by an object like Sgr with totalmass 3 x 10^10 Msun (stripped down from 5 x 10^10 Msun when it first enteredthe halo) passing through the plane.

The GALAH survey and Gaia DR2: dissecting the stellar disc's phase space by age, action, chemistry and location