Analysis of Helioseismic Power-Spectrum Diagram of A Sunspot

Abstract

The continuous high spatial-resolution Doppler observation of the Sun by Solar Dynamics Observatory / Helioseismic and Magnetic Imager allows us to compute helioseismic k-omega power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the k-omega diagram constructed by use of 40-hour observation of a stable and round sunspot. Comparing with the k-omega diagram obtained from a quiet-Sun region, inside the sunspot the f-mode ridge gets more power reduction than p-mode ridges, especially at high wavenumber. The p-mode ridges all shift toward lower-wavenumber (or higher-frequency) areas for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This probably results from acoustic waves' travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field.