A highly sensitive all‐sky electron multiplier charge‐coupled device airglow imager has been operative in Longyearbyen, Norway (78.1°N, 15.5°E), since October 2011. The imager obtains the 630.0 nm all‐sky images with an exposure time of 4 s, which is about 10 times shorter than the conventional cooled CCD airglow imagers. This new equipment allows us to image the ongoing structuring of polar cap patches in 2‐D fashion. Here we report a case in which faint undulations appeared along the trailing edge of patches propagating in the central polar cap. The separation between the fingers in the undulations was about 50–100 km and the e‐folding time of their growth was ∼5 min. We suggest that the gradient‐drift instability (GDI) is one of the possible generation mechanisms of the undulating structures. The reasons for this interpretation are (1) the asymmetry in the preference of structuring between the leading and trailing edges is qualitatively consistent with the GDI mechanism and (2) the linear growth rate of GDI calculated by using electron density estimates from simultaneous European Incoherent Scatter Svalbard radar observations is roughly consistent with the observed growth time of the fingers. Such “unstable polar cap patches” could be important sources of seed irregularities, which would eventually be broken down to smaller‐scale density perturbations affecting the transionospheric satellite communications in the central polar cap.