Everyone probably knows about the polar lights - green and purple lights that dance in the sky during geomagnetic storms. But have you ever heard of SAR arcs? Stable Auroral Red arcs were discovered in 1956 at the beginning of the space age and have been captured hundreds of times by satellite cameras since then.
Most auroral observers have never seen them because they are usually invisible to the human eye. But Matti Helin the night before was lucky enough to capture one of them in southern Finland.
“The SAR arc was visible to the naked eye for almost 30 minutes, and after disappearing for another hour and a half it remained visible to my camera. We usually see auroras in the north, but this arc appeared in our south."
This phenomenon is associated with auroras, but they are not the same thing. Regular auroras appear when high-energy particles descend along the polar lines of the magnetic field, crashing into the atmosphere (at an altitude of 100-200 km) and causing it to glow. SARs are generated differently. They are a sign of thermal energy leakage into the upper atmosphere (at an altitude of about 400 km) from the Earth's ring system of currents. Generally, SARs only become visible to the naked eye during strong geomagnetic storms. There was a G1-class storm last night, far from strong, but the arc nevertheless appeared - further proof that the auroras are full of surprises.