Panels a, c: Distance-time diagram of fractional density fluctuations in column density, $\delta N_e/\overline{N_e}$, (upper panel), and local density, $\delta n_e/\overline{n_e}$, (lower panel), within the MHD simulation. The area encircled by dashed black lines is designated for subsequent Fourier analysis. Panels b, d: Two-dimensional power spectrum of density fluctuation with respect to wavenumber and frequency. The spectra are calculated in a coordinate system aligned with the background solar wind. Gray lines represent power contours from the FFT spectrum. Thick solid and dashed lines denote nominal Alfvén and sound speeds, respectively, while a dash-dotted line marks 100 km/s. Thin lines outline the phase speed range useful for mode decomposition.