Longitude-resolved fluctuation spectrum of the pulse stack shown in Fig. 11. The FFT block size was set to 2000 rotations (2× oversampling). The panels show the normalised pulse profile (left), the LRFS (middle), the Fourier-domain modulation index (right), and the summed Fourier power on a logarithmic scale (bottom). The red-orange contours are drawn at 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95 of the maximum fluctuation PSD, and we zoomed into the area around the maximum. The lines in the bottom panel show the mean Fourier power (dashed grey line) and the 5σ power level (dotted grey line). Three significant ≥5σ features are visible that align with the leading hump in the MP’s profile and the local peak in modulation index m. We marked their centroid frequencies with red vertical lines.