We use high spatial and temporal resolution observations , simultaneously obtained with the New Vacuum Solar Telescope and Atmospheric Imaging Assembly ( AIA ) on board the Solar Dynamics Observatory , to investigate the high-frequency oscillations above a sunspot umbra .
A novel time–frequency analysis method , namely the synchrosqueezing transform ( SST ) , is employed to represent their power spectra and to reconstruct the high-frequency signals at different solar atmospheric layers .
A validation study with synthetic signals demonstrates that SST is capable to resolving weak signals even when their strength is comparable with the high-frequency noise .
The power spectra , obtained from both SST and the Fourier transform , of the entire umbral region indicate that there are significant enhancements between 10 and 14 mHz ( labeled as 12 mHz ) at different atmospheric layers .
Analyzing the spectrum of a photospheric region far away from the umbra demonstrates that this 12 mHz component exists only inside the umbra .
The animation based on the reconstructed 12 mHz component in AIA 171 Å illustrates that an intermittently propagating wave first emerges near the footpoints of coronal fan structures , and then propagates outward along the structures .
A time–distance diagram , coupled with a subsonic wave speed ( \sim 49 km s ^ { -1 } ) , highlights the fact that these coronal perturbations are best described as upwardly propagating magnetoacoustic slow waves .
Thus , we first reveal the high-frequency oscillations with a period around one minute in imaging observations at different height above an umbra , and these oscillations seem to be related to the umbral perturbations in the photosphere .