Aurora

Several terms redirect here. For other uses, see Aurora (disambiguation), Aurora Borealis (disambiguation), Aurora Australis (disambiguation), Northern Lights (disambiguation) and Southern Lights (disambiguation).

An aurora is a natural light show in the Earth's upper atmosphere. It happens when charged particles from the Sun hit atoms in the air. These hits make oxygen and nitrogen glow in colours like green, red, and purple. In places near the top and bottom of the world, called high-latitude regions, auroras are called polar lights. In the Arctic, they are called northern lights. In the Antarctic, they are called southern lights. Auroras can move in shapes like curtains, rays, spirals, and flickers. They can spread across the whole sky.

Auroras happen when the Earth's magnetosphere is disturbed. These disturbances come from faster solar wind that blows out of coronal holes and coronal mass ejections. The disturbances change the paths of charged particles moving through the plasma of the magnetosphere. Most electrons and protons fall into the upper atmosphere. This includes the thermosphere and exosphere. The falling particles precipitate into these layers. There, they cause ionization and excitation of gases in the air, which makes light in different colours and shapes. The shape of the aurora also depends on how strongly the falling particles are sped up by acceleration. The auroras form in bands around both polar regions.

Auroras are not only found on Earth. Other planets in the Solar System, brown dwarfs, comets, and some natural satellites also have auroras.

Etymology

The term aurora borealis appeared in a 1649 description by Pierre Gassendi. He wrote about an aurora that was seen across France in 1621. Gassendi had read the works of Galileo Galilei, who used the term in his many writings about auroras in 1619. The term entered the English language in 1828.

The word aurora comes from the name of the Roman goddess of the dawn, Aurora. In myths, she travelled from east to west announcing the coming of the Sun. People first used the word in English in the 14th century. The words borealis and australis come from the names of the old gods of the north wind (Boreas) and the south wind (Auster or australis) in Greco-Roman mythology. Modern style guides recommend that names of meteorological phenomena, such as aurora borealis, not be capitalised. "Auroras" is the more common plural in the U.S. However, aurorae is the original Latin plural, and scientists often use it. Sometimes aurora is used as an uncountable noun, and many sightings are called "the aurora."

Characterisation

Auroras are most often seen in the "auroral zone," a band about 67° north and south of the latitudes on Earth. The area showing an aurora at a given time is called the "auroral oval." A geomagnetic storm makes this oval grow, so auroras can be seen farther toward the equator. The northern lights, or aurora borealis, can be seen from places such as Alaska, Canada, Iceland, Greenland, the Faroe Islands, Scandinavia, Finland, Scotland, and Russia. The southern lights, or aurora australis, look almost the same. They can be seen from Antarctica, Patagonia, southeastern Australia, New Zealand, and the Falkland Islands. During the Carrington Event, the strongest geomagnetic storm ever recorded, auroras were even seen in the tropics.

Auroras can look very different. Some are a soft glow near the horizon, while others have arcs, rays, and crowns that spread across much of the sky. Some look like curtains. Their colours depend on how high they are and which gases are involved. Red appears at the highest heights, and green is the most common. Blue and purple can appear at lower heights when the Sun is very active. Auroras also change over time. During a night, they often start as glows and may grow into crowns, then fade in the reverse order. Big geomagnetic storms are most common at the peak of the 11-year sunspot cycle or in the three years after it.

Causes

Auroras are mainly caused by the solar wind interacting with Earth's magnetosphere. Scientists do not yet understand every step of this process. Solar wind particles can enter through open magnetic field lines and pass through the bow shock. Particles held in the radiation belts can also fall into the atmosphere. In the magnetotail, electrons keep leaking toward the atmosphere. Cooler electrons from the ionosphere replace them, following the second law of thermodynamics. When the solar wind is strong, it carries magnetic flux into the magnetotail. There, magnetic reconnection can release blobs called plasmoids. These "magnetic substorms" cause bursts of aurora, mostly around midnight. Very strong disturbances can even make auroras visible at middle latitudes.

Electrons that cause auroras start out trapped by magnetic field lines in the magnetosphere. They bounce back and forth as trapped particles. A magnetic mirror keeps them from reaching the atmosphere, until a change in their pitch angle lets them go. Plasma waves can cause this change, and the waves grow stronger during high geomagnetic activity. Electric fields can also speed electrons up by a few keV. Alfvén waves can speed some electrons up to keV energies. When these particles hit the upper atmosphere, photons of light are released from ionized nitrogen and oxygen atoms. This happens as the atoms return to the ground state from an excited state. The colours depend on height. Collisions can stop some light from forming, a process called collisional quenching. Green is the most common colour. Proton auroras come from hydrogen atoms. Electric currents called Birkeland currents flow through the ionosphere. Kristian Birkeland worked out that they flowed east to west along the aurora. This current was later named the auroral electrojet.

Interaction of the solar wind with Earth

Earth is always surrounded by the solar wind. This is a flow of hot plasma, which is a gas of free electrons and positive ions. The Sun sends it out in all directions. It reaches Earth at about 400 km/s. It is much weaker than Earth's own magnetic field. The interplanetary magnetic field (IMF) starts at the Sun. It is linked to sunspots and forms a spiral called the Parker spiral. Energy from the solar wind enters Earth's space through magnetic reconnection. This happens most easily when the IMF points south, toward the north magnetic pole and south magnetic pole. Auroras are more frequent and brighter when coronal mass ejections make the solar wind stronger. In the 1970s, Joan Feynman found that long-term averages of solar wind speed were linked to geomagnetic activity. She used data from the Explorer 33 spacecraft.

The solar wind shapes Earth's magnetosphere by pushing against Earth's magnetic field. This creates a barrier about 70,000 km away. It causes a bow shock further out and a magnetopause on the day side. In 2008, THEMIS probes helped scientists find the event that starts magnetospheric substorms. Geomagnetic storms that create auroras may happen more often during the months around the equinoxes.

Extraterrestrial auroras

See also: Magnetosphere of Jupiter § Aurorae

Auroras have been seen on every planet with a magnetic field. They form in the same way on each one, but the effects can be different. Saturn and Jupiter have stronger magnetic fields than Earth's. Scientists measure magnetic fields in gauss. Both planets also have large radiation belts. Auroras on these gas planets have been seen clearly by the Hubble Space Telescope, the Cassini and Galileo spacecraft, and also on Uranus and Neptune. Saturn's auroras seem to be powered by the solar wind, just like Earth's. Jupiter's auroras are more complicated. Its main glow is linked to gas from the volcanic moon Io moving through the planet's magnetosphere. Io has active volcanism and is a strong source of auroras. Europa and Ganymede have also been seen to have them.

Auroras have also been seen on Venus and Mars. On Mars, an aurora was detected on 14 August 2004 by the SPICAM instrument on Mars Express. It was in the Terra Cimmeria region. Scientists linked it to strong magnetic areas mapped with data from the Mars Global Surveyor. X-ray auroras have been seen on Mercury by the Mariner 10 and MESSENGER missions. Mercury's exosphere is so thin that these light displays happen on its surface rather than in its atmosphere. Between 2014 and 2016, the Rosetta spacecraft saw auroras around comet 67P/Churyumov–Gerasimenko at far-ultraviolet wavelengths. Scientists have suggested that exoplanets, such as hot Jupiters, might have auroras shaped by weather in their tropospheres. None has been detected yet. The first extra-solar auroras were found in July 2015 over the brown dwarf star LSR J1835+3259. They were mostly red and very bright.

Historically significant events

A Japanese diary from 1770, found in 2017, described auroras above the old Japanese capital of Kyoto. This suggests that the storm may have been 7% larger than the Carrington Event, which affected telegraph networks. The auroras from the Carrington Event on 28 August and 2 September 1859 are thought to be among the most amazing in recent history. People saw them and wrote about them in the United States, Europe, Japan, and Australia. In 1861, Balfour Stewart reported to the Royal Society about auroras recorded by a magnetograph at the Kew Observatory. The geomagnetic storm recorded there was at its strongest. From 1859 to 1862, Elias Loomis published nine papers about the Great Auroral Exhibition of 1859 in the American Journal of Science.

The aurora of 2 September 1859 is thought to have been caused by one of the strongest coronal mass ejections in history. It followed a solar flare on 1 September. This was the first time that auroras and electricity were clearly linked. Scientists could tell because of magnetometer measurements from that time, and because many telegraph lines stopped working for hours during the storm. Some lines still worked, using a geomagnetically induced current from the electromagnetic field. Operators between Boston and Portland, Maine even talked for about two hours without battery power. In May 2024, a series of solar storms made the aurora borealis visible as far south as Ferdows, Iran.

Historical views and folklore

The earliest dated record of an aurora was written in the Bamboo Annals, a historical record of ancient China, in 977 or 957 BC. The Greek explorer Pytheas described an aurora in the 4th century BC. Seneca wrote about auroras in his Naturales Quaestiones, and some people think that Pliny the Elder described one in his Natural History. The earliest picture of an aurora may be in Cro-Magnon cave paintings in northern Spain, from about 30,000 BC. In Japanese folklore, pheasants were seen as messengers from heaven. In 2020, researchers from the Graduate University for Advanced Studies and the National Institute of Polar Research said that "red pheasant tails" seen over Japan in 620 A.D. might have been a red aurora.

In the traditions of Aboriginal Australians, the aurora australis is often linked with fire. The Gunditjmara people of western Victoria called auroras puae buae ("ashes"). The Gunai people of eastern Victoria saw them as bushfires in the spirit world. The Dieri people of South Australia described an aurora as kootchee, a spirit that makes a large fire. The Ngarrindjeri people of South Australia saw the auroras over Kangaroo Island as campfires of spirits in the "Land of the Dead." Among the Māori people of New Zealand, the aurora australis was called Tahunui-a-rangi ("great torches in the sky"). People said these lights were lit by ancestors who sailed south to a "land of ice." These ancestors were said to be part of Ui-te-Rangiora's expedition, which reached the Southern Ocean.

In Scandinavia, the first mention of norðrljós (the northern lights) is in the Norwegian chronicle Konungs Skuggsjá from AD 1230. The writer offered three possible explanations. One was that the lights came from glaciers, which could store energy and become fluorescent. In 1778, Benjamin Franklin suggested that an aurora was caused by a build-up of electrical charge in the polar regions. In the Swedish city of Uppsala, Anders Celsius and Olof Hiorter watched compass needles move when an aurora appeared. Hiorter linked large magnetic changes to an aurora seen overhead. This supported their idea that "magnetic storms" cause the compass movements.

Many Native American myths also surround the aurora. The European explorer Samuel Hearne travelled with the Chipewyan Dene in 1771. The Dene believed the lights were the spirits of departed friends dancing in the sky. They thought bright lights meant their friends were very happy. During the night after the Battle of Fredericksburg, the Confederate Army saw an aurora and took it as a sign that God was on their side. The painting Aurora Borealis by Frederic Edwin Church is widely seen as representing the American Civil War.