Mapping the Galactic disk with the LAMOST and Gaia Red clump sample. II.
  3D asymmetrical kinematics of mono-age populations in the disk between 6$-14$
  \,kpc
  We perform analysis of the three-dimensional kinematics of Milky Way disk
stars in mono-age populations. We focus on stars between Galactocentric
distances of $R=6$ and 14 \,kpc, selected from the combined LAMOST DR4 red
clump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3D
asymmetrical motions of recent works and provide time tagging of the Galactic
outer disk asymmetrical motions near the anticenter direction out to
Galactocentric distances of 14\,kpc. Radial Galactocentric motions reach values
up to 10 km s$^{-1}$, depending on the age of the population, and present a
north-south asymmetry in the region corresponding to density and velocity
substructures that were sensitive to the perturbations in the early 6 \,Gyr.
After that time, the disk stars in this asymmetrical structure have become
kinematically hotter, and are thus not sensitive to perturbations, and we find
the structure is a relatively younger population. With quantitative analysis,
we find stars both above and below the plane at $R\gtrsim 9$ kpc that exhibit
bending mode motions of which the sensitive duration is around 8 \,Gyr. We
speculate that the in-plane asymmetries might not be mainly caused by a fast
rotating bar, intrinsically elliptical outer disk, secular expansion of the
disk, or streams. Spiral arm dynamics, out-of-equilibrium models, minor mergers
or others are important contributors. Vertical motions might be dominated by
bending and breathing modes induced by complicated inner or external
perturbers. It is likely that many of these mechanisms are coupled together.
