SDO/AIA images at different wavelengths. (a) Image at 19.3 nm for 2013 March 12 at 12:15 UT. The coronal hole situated close to the northern polar coronal hole is labelled CH. The coronal hole has a positive polarity, which is opposite to the expected polarity of the nearby northern polar coronal hole. During the year 2013, the polarity inversion was still in progress, so the northern polar coronal hole was very small and hard to detect (Janardhan et al. 2018). (b) Image at 19.3 nm for 2013 March 15 at 07:45 UT. The inverted sigmoid position is highlighted with the yellow box. (c) Image at 9.4 nm for 2013 March 15 at 06:03 UT. The inverted sigmoid position is again highlighted with a yellow square.