Brightness

(Section 3.2.4.3.1)

The luminance or brightness of polar aurorae is very variable. It may be faint and barely visible, or it may be comparable with that of clouds illuminated by the full Moon, and, occasionally, it may be much greater. The brightness can be estimated using the following scale:

1. Faint or weak, barely visible and about the brightness of the Milky Way, with minimal colour present
2. Similar brightness to moonlit Cirrus cloud and may have a slight greenish colour
3. Similar to the brightness of moonlit low-altitude clouds, with obvious colour
4. Strong, and bright enough to read by and to cast shadows