Temporal evolution of the wave field. Left panels: (top); (bottom) at the center of mass at the selected field lines of the flux rope at x = 0 Mm. Right panels: the periodograms of the (top) and (bottom) in time-interval 83 − 167 min. The color bar denotes the maximum initial density at each field line. The left vertical axis indicates the velocity amplitude scale. The right vertical axis denotes the vertical location of the dips of the selected magnetic field lines.