  Decoding the key dynamical processes that shape the Galactic disk structure
is crucial for reconstructing the Milky Way's evolution history. The second
Gaia data release unveils a novel wave pattern in the $L_Z-\langle V_R\rangle$
space, but its formation mechanism remains elusive due to the intricate nature
of involved perturbations and the challenges in disentangling their effects.
Utilizing the latest Gaia DR3 data, we find that the $L_Z-\langle V_R\rangle$
wave systematically shifts towards lower $L_Z$ for dynamically hotter stars.
The amplitude of this phase shift between stars of different dynamical hotness
($\Delta L_Z$) peaks around $\mathrm{2300\,km\,s^{-1}\,kpc}$. To differentiate
the role of different perturbations, we perform three sets of test particle
simulations, wherein a satellite galaxy, corotating transient spiral arms, and
a bar plus the corotating transient spiral arms act as the sole perturber,
respectively. Under the satellite impact, the phase shift amplitude decreases
towards higher $L_Z$, which we interpret through a toy model of radial phase
mixing. While the corotating transient spiral arms do not generate an
azimuthally universal phase shift variation pattern, combining the bar and
spirals generates a characteristic $\Delta L_Z$ peak at 2:1 Outer Lindblad
Resonance, qualitatively resembling the observed feature. Therefore, the
$L_Z-\langle V_R\rangle$ is more likely of internal origin. Furthermore,
linking the $\Delta L_Z$ peak to the 2:1 Lindblad resonance offers a novel
approach to estimating the pattern speed of the Galactic Bar, supporting the
long/slow bar model.
