Brightness temperature and sub-band brightness temperature difference of the FOV of the observational band 3 data at t = 290 s. (a) Brightness temperature of SB3.1 as constructed by adding the total power data to the interferometric data. (b) Difference of the brightness temperature between the sub-bands, ΔT_b|_B3 = T_b|_SB3.1 − T_b|_SB3.4 derived from the interferometric data at each sub-band. The span of ΔT_b|_B3 is between −1150 K and 670 K, but the colour scale was limited to −500 K to 500 K to reveal the small-scale structures. The locations of the two selected brightening events are marked by the green and orange markers.