The properties of rapidly changing inhomogeneities visible in the H and K lines above sunspot umbrae are described. We find as properties for these 'Umbral Flashes': (a) A lifetime of 50 sec. The light curve is asymmetrical, the increase is faster than the decrease in brightness. (b) A diameter ranging from the resolution limit up to 2000 km. (c) A tendency to repeat every 145 sec. (d) A 'proper motion' of 40 km/sec generally directed towards the penumbra. (e) A Doppler shift of 6 km/sec. (f) A magnetic field of 2100 G. (g) A decrease in this field of 12 G/sec. This decrease is probably related to the flash motion. (h) At any instant an average of 3-5 flashes in a medium-sized umbra. A weak feature often persists in the umbra after the flash. This post-flash structure initially shows a blue shift, but 100-120 sec after the flash, it shows a rapid red shift just before the flash repeats. © 1969 D. Reidel Publishing Company.
