{"id":2030,"date":"2024-02-16T16:30:02","date_gmt":"2024-02-16T16:30:02","guid":{"rendered":"https:\/\/aurorasaurudev.wpengine.com\/?p=2030"},"modified":"2026-10-29T17:04:52","modified_gmt":"2026-10-29T17:04:52","slug":"why-do-auroras-at-lower-latitudes-look-red","status":"publish","type":"post","link":"https:\/\/blog.aurorasaurus.org\/?p=2030","title":{"rendered":"Why do auroras at lower latitudes look red?"},"content":{"rendered":"

With special thanks to Dr. Janet Kozyra for her generous contributions to this post.<\/span><\/i><\/p>\n

After a big storm sends aurora to lower latitudes\u2014southward in the Northern Hemisphere, and northward in the Southern Hemisphere\u2014questions abound. One of the most common is \u201cI thought the aurora was green. Why does it look red?\u201d\u00a0<\/span><\/p>\n

Red auroras have been observed throughout history, although in the historical record it is difficult to tell which type of red aurora is which. For example, they were described as \u201c<\/span>pheasant tails<\/span><\/a>\u201d in 7th-century Japan. They have been mistaken for fire from <\/span>1st-century Rome<\/span><\/a> to the 19th century and beyond: during the famously enormous <\/span>1859 Carrington event<\/span><\/a>, people saw blood-red skies as far south as the Caribbean, and fire crews were sent to look for forest fires on the horizon.\u00a0<\/span><\/p>\n

\"A<\/a>
Eighteenth-century drawing of red aurora from Tohoku University Library, ff. 26b-27a (at Kyoto), Japan. Translated original text begins: \u201cOn 1770 September 17, at night, it got totally reddened within the sky beyond the northern mountains. Those who saw thought it was a great conflagration.\u201d Image and text from Hayakawa et al., 2017<\/a>.<\/figcaption><\/figure>\n

As technology advances, <\/span>cameras<\/span><\/a> are becoming increasingly good at capturing faint features in red auroras. During the Halloween Storm of 2003, <\/span>blood-red aurora<\/span><\/a> were photographed as far south as Texas in the United States, and photographers caught red aurora during the <\/span>March 23<\/span><\/a>, <\/span>April 23<\/span><\/a>, and <\/span>November 5, 2023<\/span><\/a> storms.\u00a0<\/span><\/p>\n

There can be four ways for auroras at lower latitudes to glow red: one involves regular auroras, and the others involve special, rare events. In this post, we\u2019ll introduce all these types, our valentines from the sky!\u00a0<\/span><\/p>\n

With regular auroras, location is everything<\/span><\/h2>\n
\"Diagram<\/a>
Diagram by Aurorasaurus, CC BY-SA<\/figcaption><\/figure>\n

The aurora dances high in the upper atmosphere. Particles from the Earth\u2019s magnetic field fill atoms and molecules with extra energy that the atoms and molecules give off as light. Because of the way atoms and molecules are layered in the atmosphere, the colors in auroras are also layered. The highest, least dense area, which has oxygen atoms, shines red, and stretches above 120 miles (200km) up.\u00a0<\/span><\/p>\n

When something is very high in altitude, it is easier to see from a greater distance. So the taller the aurora, the further away it can be seen. At the same time, the surface of the Earth curves, blocking part of the view. So even if an aurora is especially tall and bright, from far away you may only see the top of it: the high-altitude red over the horizon.\u00a0<\/span><\/p>\n

Great Red Auroras are distinct, storm-time phenomena<\/span><\/h2>\n
\"Above<\/a>
A rare red aurora over Alaska in February 1958. University of Alasaka Geophysical Institute photo, featured in their blog post \u201cRed aurora rare enough to be special<\/a>\u201d.<\/figcaption><\/figure>\n

Great Red Auroras (also called \u201cType A\u201d aurora<\/a>) tend to occur with some particularly intense storms as bright, short-lived phenomena at the peak of the storm. We don\u2019t know exactly why, but scientists have <\/span>some ideas<\/span><\/a>. Great Red Auroras are associated with large amounts of low-energy electrons. When these electrons enter the upper atmosphere, most don\u2019t have enough energy to go deeper than the uppermost, red layer. So they light up <\/span>only<\/span><\/i> that uppermost layer of oxygen atoms. In addition, this intense rain heats up the upper atmosphere, changing the chemistry that creates light. The ruby color created is as pure as a blood red laser pointer, and unusually bright. Because they are so high up, Great Red Auroras are easier to see from far away than green aurora. Sometimes, Great Red Auroras can have a bit of green beneath them, but like regular auroras like these are hard to see at great distances because they are blocked by the curve of the Earth.\u00a0<\/span><\/p>\n

SAR Arcs are lower latitude and dimmer<\/span><\/h2>\n
\"Peering<\/a>
SAR arc photographed by Bob King on September 26, 2022, at 9:15 p.m. He writes, “While unmistakable in the camera, it was only fleetingly visible with the naked eye.” Exposure: 30 seconds, ISO 2000, f\/2.8, 12mm lens. Featured in his article, “Astro Bob: Night of the bloody red arc \/ Bullseye for DART!<\/a>” and used with permission.<\/figcaption><\/figure>\n

The third low-latitude red auroras are called Stable Auroral Red arcs, or SAR arcs. Most of the time these are diffuse, dim, and invisible to the human eye\u2014so much so that they weren\u2019t <\/span>studied until 1956<\/span><\/a>. With increasingly sensitive consumer cameras, though, participatory scientists are more empowered than ever before to catch them! In fact, the \u201cStable\u201d part of its name came from early observations in which it didn\u2019t change much, but now that there are more observations that understanding is evolving. During particularly intense storms, they can even become bright enough to be seen by the unaided eye.\u00a0<\/span><\/p>\n

Like the other two types, the red of SAR arcs is made from high-altitude oxygen, but the mechanics that bring it about are different and have to do with heat and chemistry in the upper atmosphere and the inner part of the Earth\u2019s magnetic field. A SAR arc occurs equatorward of the aurora, in a band that stretches east to west. <\/span>Sound familiar<\/span><\/a>, STEVE fans? There has even been <\/span>a study<\/span><\/a> on SAR arcs evolving into STEVEs! Dr. Carlos Martinis, the lead author, gave a presentation on the study to the Aurorasaurus Ambassadors, watchable <\/span>online<\/span><\/a> on the Aurorasaurus YouTube channel. That said, SAR arcs are wider and more red than STEVEs, and that\u2019s two ways to tell them apart.\u00a0<\/span><\/p>\n

Equatorial Red Airglow is elusive and exciting<\/span><\/h2>\n
\"\"<\/a>
Equatorial aurora photographed at the European Southern Observatory (ESO)\u2019s Paranal Observatory in Chile on September 29, 2011 by Y. Beletsky. Image and caption from He et al., 2020<\/a>.<\/figcaption><\/figure>\n

Sometimes, a low-latitude red glow in the night sky might not be aurora after all. Especially during strong solar storms, historical accounts from ships, balloons, and airplanes flying through equatorial regions have noted visible red and green glowing lights in the sky. These lights have also been photographed from observatory sites, and are a different phenomenon called \u201cairglow\u201d that is caused by chemistry in the upper atmosphere and natural quirks in the Earth\u2019s magnetic field (<\/span>He et al., 2020<\/span><\/a>). Airglow can also be enhanced, or made more intense, by geomagnetic storms. Studies are ongoing, and include exciting analyses of <\/span>historical observations<\/span><\/a> in equatorial areas like Korea and China. So if you are in equatorial regions and photograph the sky glowing red at night, you can help build datasets for everyone to learn more by making an Aurorasaurus.org report.<\/span><\/p>\n

Which is which?<\/span><\/h2>\n

It is difficult to tell whether what one is seeing from a low latitude is high-altitude regular aurora, Great Red Aurora, or SAR arcs. (Equatorial red airglows generally only occur in special, equatorial locations.) Some can also occur at the same time: Great Red Auroras and SAR arcs have been photographed together. If you\u2019ve photographed <\/span>any <\/b>red aurora, you can make a report to Aurorasaurus.org<\/a> and compare it with others from your area and across the world during the same storm.\u00a0<\/span><\/p>\n

Here are some special features that may help tell them apart, with many thanks to our colleague Dr. Janet Kozyra:<\/span><\/p>\n

Timing<\/b>:\u00a0<\/span><\/p>\n