tokens: the divergence of velocity shown in panels (a) and (b) for a short period of time to display the continuous generation of wave trains. (An animation of this figure is available.) In order to study the dynamical behavior of the wave trains outside the CS, we plot the time–distance diagram of the normalized running difference of the density Δ ρ / ρ along the line x = −0.15 L from t = 857.9 to 2038.6 s in Figure 8 (a). There are violent variations below the edge of the CME, and they may have quasiperiodic behavior indicating the propagation of the coronal wave trains. Since the flux rope keeps rising, it is impossible to fix a point aside to check the periodicity of the wave trains. The estimation of the periodicity is also sensitive to the chosen line, along which the calculation is executed. For instance, we followed the dashed line almost parallel to the edge of the CME from panel (a). The associated distribution shows periodicity in panel (b), and the period is calculated to be ∼76.3 s. Therefore, we directly investigate the evolution of the turbulent region above the upper TS. Figure 8. (a) Normalized running difference of the density along the line x = −0.15 L extracted from t = 857.9 to 2038.6 s. The blue diagonal line corresponds to the low-density wing to the left of the TS in Figure 2 (f), and the red diagonal line is where the density jumps back to the coronal value outside the flux rope. (b) Associated distribution along the dashed line in panel (a). The symbol Δ ρ / ρ is defined as ( ρ ( t ) − ρ ( t − Δ t ))/ ρ ( t ). A typical Δ t ≈ 9.2 s. Takasao Shibata ( 2016 ) described how the CS outflow can form a blob of gas between the sides of a “tuning fork” and how the backflow of the gas drives an oscillation of the magnetic structure. The oscillation can drive waves that propagate well outside the “buffer region” where it occurs. Figures 7 (c) and (d) show the dynamics of the backflow. The relation between the coronal wave trains and the turbulence under the CME is analyzed in Figure 9 . In panel (a), we show the time evolution of the maximum of the horizontal velocity V x of the backflow and the fast magnetosonic Mach number before the upper TS. Note that the Mach number is defined as the ratio of the flow velocity to the fast-mode magnetosonic speed. The max( V x ) decreases in general, indicating that the coronal wave trains may appear at different speeds, which could be caused by variations in the local plasma density and magnetic strength. The peaks reveal the oscillation of the MTF. Interestingly, one can find that the Mach number mainly varies around 2.4, which is larger than that before the TS above the flare loops (1.5–1.8 found in observations and simulations; Chen et al. 2015 ; Ye et al. 2019 ). This reveals that it is easier to form the upper TS than the lower one, and the variation of the Mach number is related to the interplay between plasmoids and the TS (Shen et al. 2018 ). Then, we performed the Morlet transform on
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