Recent observations of sunspot ’ s umbra suggested that it may be finely structured at a sub-arcsecond scale representing a mix of hot and cool plasma elements .
In this study we report the first detailed observations of the umbral spikes , which are cool jet-like structures seen in the chromosphere of an umbra .
The spikes are cone-shaped features with a typical height of 0.5-1.0 Mm and a width of about 0.1 Mm .
Their life time ranges from 2 to 3 min and they tend to re-appear at the same location .
The spikes are not associated with photospheric umbral dots and they rather tend to occur above darkest parts of the umbra , where magnetic fields are strongest .
The spikes exhibit up and down oscillatory motions and their spectral evolution suggests that they might be driven by upward propagating shocks generated by photospheric oscillations .
It is worth noting that triggering of the running penumbral waves seems to occur during the interval when the spikes reach their maximum height .