Temporal evolution of a solar UV burst observed by IRIS on 2013 October 22. The temporal variation of the intensity in the IRIS 1400 Å channel essentially showing the light curve in the Si IV line is plotted in panel a. Panel b: immediate surroundings of the UV burst as seen in the 1400 Å channel (reverse linear intensity scaling) at the time indicated by the vertical blue line in panel a. The box in panel b surrounding the UV burst outlines the region where we integrated the emission to derive the light curve in panel a. The line-of-sight component of the magnetic field based on HMI is shown in panel c with a linear scaling of the magnetic field in the range ±200 G. The location of the UV burst is over-plotted with a red contour of the 1400 Å channel intensity. This only appears between a small negative (dark) magnetic polarity and a major opposite (white) polarity. The blue arrow indicates the direction of motion of the minor negative-polarity feature that causes the UV burst. See Sect. 4.2 for details.