Stable auroral red (SAR) arc emission observed over North America from space by the Dynamics Explorer 1 satellite. This image was filtered at 6300 angstroms. the characteristic emission of SAR arcs.
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Image provided courtesy of L. Frank and J. Craven from the Dynamics Explorer 1
The SAR Arc
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.
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