Various small-scale structures abound in the solar atmosphere above active regions , playing an important role in the dynamics and evolution therein .
We report on a new class of small-scale transition region structures in active regions , characterized by strong emissions but extremely narrow Si iv line profiles as found in observations taken with the Interface Region Imaging Spectrograph ( IRIS ) .
Tentatively named as Narrow-line-width UV bursts ( NUBs ) , these structures are located above sunspots and comprise of one or multiple compact bright cores at sub-arcsecond scales .
We found six NUBs in two datasets ( a raster and a sit-and-stare dataset ) .
Among these , four events are short-living with a duration of \sim 10 mins while two last for more than 36 mins .
All NUBs have Doppler shifts of 15–18 km s ^ { -1 } , while the NUB found in sit-and-stare data possesses an additional component at \sim 50 km s ^ { -1 } found only in the C ii and Mg ii lines .
Given that these events are found to play a role in the local dynamics , it is important to further investigate the physical mechanisms that generate these phenomena and their role in the mass transport in sunspots .