This is an attempt to compare the visual view (before dark adaptation) of the aurora with the camera's view (move your mouse on and off the photo). Because the eye's sensitivity to color is poor in the dark, aurora tends to look a pale green, even when the activity is moderately high. More often than not, it's hard to distinguish between aurora and high clouds. In very high activity, some other colors (e.g. red) may be visible to the eye.
For this reason, it can be misleading to simply glance outside to check to see if auroral activity is in progress. A camera should always be used for a thorough check. In addition, because the auroral activity can change rapidly and significantly in just a few minutes, a once-per-hour check is not adequate either. If you're serious about photographing the aurora, it's an all-night effort!