The aurora we are most familiar with is the polar aurora. This is what people are referring to when they speak of the northern or southern lights. But there are other less-known auroral activity, such as SAR arcs. The SAR arcs or Stable Auroral Red arcs have been viewed with interest since their discovery in 1956.

The reason that SAR arcs were discovered so late compared to poleward aurora is that the SAR arc emission occurs very rarely in the visible spectrum. So, optical instruments are required to register their presence and note the frequency with which they occur. SAR arcs are seen as relatively featureless, slowly changing bands of red light that can extend over the entire night sky. They can appear simlutaneously in both the northern and southern hemispheres.

SAR arcs occur on approximately 10-12% of nights throughout the solar cycle. The long-lived, soft, red glow of the SAR arcs most likely reflects the slow energy loss from ring current ions as they bounce back and forth along the Earth's magnetic field lines. Although the exact sequence of physical processes that are the energy source for the SAR arcs is still debated by scientists today, SAR arc emission does increase with increasing solar activity.

The aurora we are most familiar with is the polar aurora. This is what people are talking about when they say the northern or southern lights. But there are other less-known aurora, such as SAR arcs. The SAR arcs or Stable Auroral Red arcs were discovered in 1956.

The reason that SAR arcs were discovered so late compared to polar aurora is that SAR arcs aren't usually visible. So, humans have to use instruments to tell that they are there (see image to left). SAR arcs can stretch across the entire sky.

Scientists have only known that SAR arcs existed for last 50 years. So, there are still debates as to exactly what produces SAR arcs. It is likely that this faint red glow is caused by the release of energy by the Earth's ring current system.

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!"