Comparing the Sun Watcher Using Active Pixel System Detector and Image Processing Instrument to the Atmosphere Imaging Assembly Instrument Through Measurements of Polar Coronal Holes

The PRoject for OnBoard Autonomy 2/Sun Watcher using Active pixel system detector and image Processing (PROBA2/SWAP) instrument images the full-disk extreme ultraviolet (EUV) Sun using a complementary metal-oxide semiconductor active-pixel sensor (CMOS-APS) detector with a filter centered on a 174 Å passband at a cadence of one to two minutes. In contrast, the Atmosphere Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) has a passband filter centered on 171 Å and uses a charge-coupled device (CCD) detector to make full-disk observations of the EUV corona. The images that these two telescope designs produce are visually quite similar in active regions, coronal loops, and the quiet corona. This work takes a deeper look at the stability of the most difficult coronal features to capture in an image: polar coronal holes. Polar coronal holes are the longest-lived features on the Sun and are critical to understand the global state of the solar corona, but because of an oblique viewing angle, obstruction due to the coronal plasma scale height and lack of ground-truth magnetic-field measurements make reliable segmentation of polar holes difficult. We use perimeter tracing to make consistent measurements of a polar-hole’s perimeter and area in both SWAP 174 Å and AIA 171 Å images. The generated time series of coronal-hole parameters rarely agree with each other. Direct comparison of polar-hole measurements generated by these two imagers allows us to simultaneously analyze the physical properties of polar coronal holes and to identify systematic differences between the two different instruments.