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We investigate the three-dimensional asymmetrical kinematics and present timestamps of the Milky Way disk between Galactocentric distances of $R=12$ and 15\,kpc, using red clump stars selected from the LAMOST Galactic survey, alsowith proper motion measurements provided by the Gaia DR2. We discover velocitysubstructure above the Galactic plane corresponding to a density dip foundrecently ("south-middle opposite" density structure[R $\sim$ 12-15 \,kpc, Z$\sim$ 1.5 \,kpc] discovered in \citet{Wang2018b, Wang2018c}) in the radial andazimuthal velocity. For the vertical velocity, we detect clear vertical bulkmotions or bending mode motions, which has no clear north-south asymmetrycorresponding to the in-plane asymmetrical features. In the subsample of starswith different ages, we find that there is little temporal evolution of thein-plane asymmetry from 0$-$14 \,Gyr, which means the structure is sensitive tothe perturbations in almost cosmic time possibly. We propose that the possiblescenario of this asymmetric velocity structure is caused by the mechanismsgenerated in-plane, rather than vertical perturbations.

astrophysical journal/the astrophysical journal

Mapping the Galactic disk with the LAMOST and Gaia Red clump sample:  III: A new velocity substructure and time stamps of the Galactic disk  asymmetry in the disk between 12$-$15 \,kpc