Here are X-rays images (shown on the same brightness scale) of the north polar region obtained by Chandra HRC-I on different days, showing large variability in soft (0.1-10.0 keV) X-ray emissions from Earth s aurora. Note that the images are not snap shots, but are approximately 20-min scans of the northern auroral region in the HRC-I field-of-view. The brightness scale in Rayleighs (R) assumes an average effective area of 40 cm2. The day-night terminator at an altitude of 0 km is displayed with lighting. The day-night terminator at an altitude of 100 km is shown by the blue line.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 18.104.22.168.0