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 blue shifted umbral wave (UW) and red-shifted plasma returning to the
initial state. Here we use 5 s cadence images acquired at ±0.04~nm off the
Halpha line center by the Visible Imaging Spectrometer (VIS) installed on the
Goode Solar Telescope (GST) to detect the origin of UFs and UWs in a sunspot
with a uniform umbra free of LBs 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 Halpha-0.04 nm intensity. GST magnetic field
measurements using Near Infra-Red Imaging Spectropolarimeter (NIRIS) also
showed that the dominant location of prevalent UF origin is co-spatial
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 can not 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 sub-surface 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.Comment: 9 pages, 5 figure
