Temporal evolution of the brightness temperatures at of the two selected brightenings in the observational data. Left column: Event 1. Right column: Event 2. The locations of the events are indicated in the FOV of Fig. 11. (a,d) Brightness temperatures of SB3.1 and SB3.4. (b,e) Brightness temperature differences between the sub-bands ΔT_b|_B3. The green graph shows the original data, and the black curve schows a boxcar average over 10 s. (c,f) Time derivative of the (boxcar-averaged) sub-band differences.