EUI brightening (E-1). Panel a: HRI_EUV image taken on 23 February 2021 at 17:17:28 UT, at about the time when the intensity of the EUI brightening peaked. The smaller dashed box shows the region covered by one SPICE-binned pixel (4″ × 4.392″) when the slit was at the second spatial step of the short raster. The O VI intensity at this pixel (blue diamonds) in panel b is calculated from Gaussian fits to the line in the SPICE data. The error bars were calculated from the uncertainties of the fitting parameters. This region was also used to calculate the HRI_EUV intensity (dashed line) in panel b. The larger solid box shows the region we used to compute the light curve (solid line) shown in panel b. The horizontal bar in the lower left corner shows the SPICE exposure duration, which is about 4.7 s. An animation of panel a is available online.