What is this unusual red halo surrounding this aurora? It is a stable red auroral arc, called SAR in English, which is an abbreviation of the group of words Stable Auroral Red. These SAR arcs are rare and have only been recognized and studied since 1954. The wide-angle image shown here, which captures almost a completely stable auroral arc surrounding the more common green and red aurora, was taken in early December 2023 from Bolburn, New Zealand, during a storm Especially strong geomagnetism. The reason behind the formation of stable auroral arcs remains a subject of research, but is likely related to Earth's magnetic field, generated by molten iron flowing deep into our planet. This magnetic field generally redirects charged particles from the solar wind toward the poles. However, it also traps a ring of ions near the equator, where they can gain energy from the magnetosphere during high solar activity. Energetic electrons in this ion ring can then collide with oxygen in Earth's ionosphere higher than in a typical aurora and excite it, causing the oxygen to glow red. Ongoing research has discovered that SAR can turn into purple and green STEVE.
Astronomical Picture of the Day (APOD)
Translated by Didier Jamet