Impulsive heating in the core of AR 12665. Panels a–d: snapshots of various observables in the core close to the peak in Fe XVIII intensity (see Fig. 1). Maps of AIA 94 Å Fe XVIII (panel a), AIA 171 Å (panel b), ratio of AIA 1600 Å to 1700 Å (UV ratio; panel c), and with a grey-scale an HMI line-of-sight magnetic field map (panel d) are displayed. The magnetic flux density is saturated at ±300 G. White (dark) shaded regions represent positive (negative) magnetic polarity. Panel a: E and W label eastern and western footpoints of the loop. The red contour outlines a contiguous patch in the UV ratio map detected by our method. The plus symbol marks the centroid of the contoured region. The circle with radius ≈5.4 Mm, about the centroid, outlines the region used for magnetic analysis. A 10 Mm scale is overlaid. Panels a and d: yellow coloured curves represent magnetic field lines traced from a linear force free field extrapolation. Panels e–g: time series of various observables. Panel e: disk-integrated GOES 1−8 Å X-ray flux (blue), Fe XVIII emission integrated over the AR’s core (black), UV ratio (red) and AIA 171 Å (maroon) emission integrated over the contoured region at the western footpoint. Panels f and g: integrated magnetic flux as a function of time of the dominant (positive) and minor (negative) magnetic polarities within the circled region overlaid on panel d. The dashed line is a linear fit to the magnetic flux curve, with its slope quoted in the top right corner. Panel h: UV ratio and AIA 171 Å emission from the eastern (E) footpoint regions marked in panel a. The vertical dotted line in panels e–h marks the peak of Fe XVIII as identified in the 120 s cadence data. For display purposes, in panels e and h we plot AIA data at their native cadence (12 s EUV and 24 s UV). Animation of panels a–g is available online. See Sects. 2, 3.1 and Appendix A for details.