KUNMING -- Chinese researchers have recently studied the structure of chromospheric umbral waves in sunspots and discovered the driving source of umbral waves, according to the Yunnan Observatories under Chinese Academy of Sciences.

Oscillations are ubiquitous in sunspots and the associated higher atmospheres. However, it is still unclear whether these oscillations are driven by the external acoustic waves or generated by the internal magnetoconvection.

Researchers studied the fine structure of chromospheric umbral waves in two sunspots by using the Interface Region Imaging Spectrograph and data from other telescopes.

They found for the first time that two one-armed spiral wave patterns coexist in the umbra, and they can rotate either in the same or opposite directions.

Furthermore, by analyzing the spatial distribution of the oscillation centers of the one-armed spiral wave patterns within the umbra, they found that the chromospheric umbral waves repeatedly originate from the regions with high oscillation power and most of the umbral waves occur in the dark nuclei and strong magnetic field regions of the umbra.

The study results indicate that the chromospheric umbral waves are likely reacting to the external acoustic waves oscillations.

The study has been published in the journal Solar Physics.