Spatial Distribution of the Origin of Umbral Waves in a Sunspot Umbra

Umbral flashes (UFs) are emissions in the core of chromospheric lines caused by upward propagating waves steepening into shocks. UFs are followed by an expanding blueshifted umbral wave and redshifted plasma returning to the initial state. Here we use 5 s cadence images acquired at ±0.04 nm off the H line center by the Visible Imaging Spectrometer installed on the Goode Solar Telescope (GST) to detect the origin of UFs and umbral waves (UWs) in a sunspot with a uniform umbra free of light bridges and clusters of umbral dots. The data showed that UFs do not randomly originate over the umbra. Instead, they appear to be repeatedly triggered at locations with the lowest umbral intensity and the most powerful oscillations of H –0.04 nm intensity. GST magnetic field measurements using the Near-Infrared Imaging Spectropolarimeter also showed that the dominant location of prevalent UF origin is cospatial associated with the strongest fields in the umbra. Interface Region Imaging Spectrograph 149.0 nm images showed that no bright UV loops were anchored in the umbra in general, and near the UF patches in particular, suggesting that UFs and UWs alone cannot be responsible for the origin of warm coronal loops. We thus conclude that the existence of locations with prevalent origin of UFs confirms the idea that they may be driven by a subsurface source located near the axis of a flux rope, while the presence of several UFs trigger centers may indicate the complex structure of a sunspot umbra.