Umbral flashes and their association with running penumbral waves: a study using MAST Ca II 8542 Å narrow-band observations

Umbral flashes (UFs) are one of the most dynamic phenomena observed in the sunspot umbra at the chromospheric heights. In this paper, we present spectroscopic observations of UFs in the Ca II 8542 Å line recorded by a narrow-band imager working with the Multi-Application Solar Telescope (MAST). The deduced data are analysed to obtain various properties of the UFs occuring at different locations inside the umbral boundary. An intensity enhancement of up to 30% or more was observed at the location of UFs, with a periodicity ≈3 min. The line-of-sight (LOS) velocity of UFs was estimated using bisector application to the emission profile resulting from the removal of mean umbral and the mean quiet Sun (QS) line profiles. The emission profiles resulting from removing the mean umbral profile were observed to better represent the emission component of the UF line profile. Both up-flows and down-flows of the order ≈5 km s⁻¹ were associated with the UFs with an average up-flow of ≈1 km s⁻¹. Out of all UFs analysed, 31% were observed to be associated with down-flows in case of removal of the mean umbral profile from the UF line profile. We observed multiple radially propagating LOS velocity disturbances (≈20,40 km s⁻¹) in the penumbra, which might be associated with the UFs, even though we could not establish a one-to-one correspondence. The horizontally propagating LOS velocity disturbances could produce the visual effect of running penumbral waves, which produce intensity fluctuations in intensity images when observed at the line-centre wavelength. The simultaneous photospheric HMI observations showed no distinct intensity or velocity signatures corresponding to the UFs observed in the chromospheric Ca II 8542 Å line.