  Sunspot atmospheres are highly inhomogeneous hosting both quasi-stable and
transient features, such as `umbral micro-jets' and dark fibril-like events. We
seek to understand the morphological properties and formation mechanisms of
small-scale umbral brightenings (SSUBs; analogous to umbral micro-jets) and to
understand whether links between these events and short dynamic fibrils, umbral
flashes, and umbral dots can be established. An SST filtergram time-series
sampling the Ca II H line and a CRISP full-Stokes 15-point Ca II 8542 A line
scan dataset were used. The average lifetime and lengths of 54 SSUBs identified
in the sunspot umbra are found to be 44.2 seconds (sigma=20 seconds) and 0.56"
(sigma=0.14"). The spatial positioning and morphological evolution of these
events was investigated finding no evidence of parabolic or ballistic profiles
nor a preference for co-spatial formation with umbral flashes. The presence of
Stokes V profile reversals provided evidence that these events could form
through the development of shocks in the chromosphere. The application of the
weak-field approximation indicated that changes in the line-of-sight magnetic
field were not responsible for the modifications to the line profile and
suggested that thermodynamic effects are the actual cause of the increased
emission. Finally, a sub-set of SSUBs were observed to form at the foot-points
of short dynamic fibrils. Overall, we found no correlation between the spatial
locations where these events were observed and the occurrence of umbral dots
and umbral flashes. SSUBs, however, have lifetimes and spectral signatures
comparable to umbral flashes and are located at the footpoints of short dynamic
fibrils, during or at the end of the red-shifted stage. It is possible,
therefore, that these features form due to the shocking of fibrilar material in
the lower atmosphere upon its return under gravity.
