Mysterious Blood-Red Arc Appears in Scandinavian Sky.
A rare stable auroral red arc (SAR) appeared in the sky above Denmark on March 23, caused by oxygen molecules in the upper atmosphere becoming superheated by Earth's ring current system during a powerful geomagnetic storm triggered by a coronal mass ejection from the sun. SARs occur frequently but are normally invisible to humans, and only become visible when strong solar storms weaken the magnetosphere. The SAR coincided with the most powerful geomagnetic storm to hit Earth for six years and was not the only unusual light show observed during the recent solar storm, with the aurora-like phenomenon STEVE also visible across the United States and parts of the United Kingdom.