Sunspot oscillations consist of multiple wave modes, making it challenging to isolate individual physical processes. To decompose these multi-modal oscillations, we apply empirical mode decomposition (EMD) to Doppler velocity data obtained with the Fast Imaging Solar Spectrograph. By avoiding arbitrary frequency filtering, EMD resolves the oscillations into four distinct modes with unique periodicities and spatial distributions: c₁ in the umbra (periods of ∼ 2 min) ; c₂ in the umbra and penumbra (2. 5-4 min) ; c₃ in the outer penumbra (4-6 min) ; and c₄ in the superpenumbra (∼ 10 min). We find that all modes, including the high-frequency 1-minute oscillations, coexist in the superpenumbral fibrils and represent potential candidates for transverse waves. Analysis of the c₁ and c₂ modes reveals the coexistence of umbra-trapped body waves and running penumbral waves within the umbra. Meanwhile, the c₃ and c₄ modes demonstrate that sunspot waves undergo significant nonlinear evolution as they propagate through the stratified atmosphere. Our results indicate that EMD effectively classifies dispersive oscillations into distinct groups based on their intrinsic timescales and underlying physical natures.
Kang et al. (Thu,) studied this question.