Signature of high-order azimuthal MHD body modes in sunspot's low atmosphere

The five-minute oscillations inside sunspots appear to be the absorption of the solar p-mode. It is a potential tool to probe a sunspot's sub-structure. We studied the collective properties of five-minute oscillations in the power and phase distribution at the sunspot's umbra-penumbra boundary. The azimuthal distributions of the power and phase of five-minute oscillations enclosing a sunspot's umbra were obtained with images taken with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). The azimuthal modes were quantified with periodogram analysis and justified with significance tests. The azimuthal nodal structures in an approximately axially symmetric sunspot AR 11131 (2010 Dec 08) were investigated. Mode numbers m = 2,3,4,7,10 were obtained in both 1700 Å and 1600 Å bandpasses. The 1600 Å channel also revealed an extra mode at m = 9. In the upper atmosphere (304 Å), fewer modes were detected at m = 3, 4, 7. The azimuthal modes in the sunspot's low atmosphere could be interpreted as high-order azimuthal MHD body modes. They were detected in the power and phase of the five-minute oscillations in sunspot AR 11131 with SDO/AIA data. Fewer modes were detected in the sunspot's upper atmosphere.