This is a SAR arc observed over North America from space by the Dynamics Explorer 1 satellite.
Click on image for full size
Image provided courtesy of L. Frank and J. Craven from the Dynamics Explorer 1
The SAR Arc
If someone says they saw an aurora, you might picture something like this. There is another type of aurora that we can't see. These aurora are called SAR arcs. The SAR stands for Stable Auroral Red. That basically means that pictures taken of these aurora by special instruments (remember, we can't see these aurora) show a soft glow of red light.
SAR arcs happen about 10% of all nights. So if you're really smart, you can say to your parents while looking up at the night sky, "hey, what's that?" When they don't answer because they don't see anything special, you can say, "maybe it's just an invisible SAR arc!"
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