Alignment

The division of the data into two blocks was vital to compensate for solar differential rotation in the dataset (estimated as 1 over each 167 image block). Initially all wavelengths were aligned by calculating the offsets of the middle image of each block to the corresponding 1600 Å image, and shifting all images in each wavelength correspondingly. At each wavelength, all images were then aligned to the middle image of the timeseries using cross-correlation programs available in SSW. Any large offsets were replaced by the average of the preceding and following offsets, and the cross-correlation repeated until all offsets were less than 0.2 pixels (i.e., 0.1). This procedure was carried out for each block separately. Finally, the offset of the middle image of block A to block B was found using the same cross-correlation procedure and all block B data were shifted by this offset. This resulted in the entire timeseries, in all four UV wavelengths, being aligned to within 0.1. Due to solar rotation, several pixel rows and columns were not always present in the data, so that the aligned data FOV was reduced from 256 x 640 pixels² to 215 x 600 pixels².