An aurora ( aurorae or auroras) is a natural light display in Earth's upper atmosphere caused by charged particles from the Sun colliding with atoms in the atmosphere. These collisions excite oxygen and nitrogen, which then emit light of different colours such as green, red, and purple. When observed in high-latitude regions they are called polar lights and aurora polaris. In the Arctic they are called the northern lights or aurora borealis; in the Antarctic, the term southern lights or aurora australis is used. Auroras display dynamic patterns of radiant light that appear as curtains, rays, spirals or dynamic flickers covering the entire sky.Lui, A., 2019. Imaging global auroras in space. Light: Science & Applications, 8(1).
Auroras are the result of disturbances in the Earth's magnetosphere caused by enhanced speeds of solar wind from coronal holes and coronal mass ejections. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere). The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles.
Other planets in the Solar System, brown dwarfs, comets, and some natural satellites also host auroras.
Etymology The term aurora borealis appeared in a 1649 description by Pierre Gassendi of an auroral display visible all over France in 1621. Gassendi had read the works of Galileo Galilei, who used the term in his extensive writings about aurora in 1619. On p. 39, Galileo explains that auroras are due to sunlight reflecting from thin, high clouds. From p. 39: ("... many of you will have seen, more than once, the sky in the night hours, in parts towards the north, illuminated in a way that the clear [sky] does not yield to the brighter aurora, far from the rising of the sun; an effect that, by my thinking, has no other origin than being part of the vaporous air that surrounds the Earth, for some reason thinner than usual, which, being sublimated far more than expected, has risen above the cone of the Earth's shadow, so that its upper part, being struck by the sun['s light], has been able to reflect its splendor and to form this aurora borealis.") The term entered the English language in 1828.
The word aurora is derived from the name of the Roman goddess of the dawn, Aurora, who travelled from east to west announcing the coming of the Sun. Aurora was first used in English in the 14th century. The words borealis and australis are derived from the names of the ancient gods of the north wind (Boreas) and the south wind (Auster or australis) in Greco-Roman mythology.
Grammar Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized. The term "auroras" is the more common plural in the U.S.; however, aurorae is the original Latin plural and is often used by scientists. In some contexts, aurora is an uncountable noun, multiple sightings being referred to as "the aurora".
Characterisation
Auroras are most commonly observed in the "auroral zone", a band approximately 6° (~660 km) wide in latitude centred on 67° north and south. The region that currently displays an aurora is called the "auroral oval". The oval is displaced by the solar wind, pushing it about 15° away from the geomagnetic pole (not the geographic pole) in the noon direction and 23° away in the midnight direction. The peak equatorward extent of the oval is displaced slightly from geographic midnight. It is centred about 3–5° nightward of the magnetic pole so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun, which is called magnetic midnight.
Early evidence for a geomagnetic connection comes from the statistics of auroral observations. Elias Loomis (1860), and later Hermann Fritz (1881) and Sophus Tromholt (1881) in more detail, established that the aurora appeared mainly in the auroral zone.
In northern latitudes, the effect is known as the aurora borealis or the northern lights. The southern counterpart, the aurora australis or the southern lights, has features almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. The aurora australis is visible from high southern latitudes in Antarctica, Patagonia, southeastern Australia, New Zealand, and the Falkland Islands. The aurora borealis is visible from areas around the Arctic such as Alaska, Canada, Iceland, Greenland, the Faroe Islands, Scandinavia, Finland, Scotland, and Russia. A geomagnetic storm causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes or higher in the south. On rare occasions, the aurora borealis can be seen as far south as the Mediterranean, East Asia, and the southern states of the US, while the aurora australis can be seen as far north as New Caledonia, South Africa, the Pilbara region in Western Australia, and Uruguay. During the Carrington Event, the greatest geomagnetic storm ever observed, auroras were seen even in the tropics.
Auroras seen within the auroral oval may be directly overhead. From farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs, which can be subvisual. Auroras are occasionally seen in latitudes below the auroral zone when a geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most common during the peak of the 11-year sunspot cycle or during the three years after the peak. An electron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to the local geomagnetic field vector B. This angle is known as the "pitch angle" of the particle. The distance, or radius, of the electron from the field line at any time is known as its Larmor radius. The pitch angle increases as the electron travels to a region of greater field strength nearer to the atmosphere. Thus, it is possible for some particles to return, or mirror, if the angle becomes 90° before entering the atmosphere to collide with the denser molecules there. Other particles that do not mirror enter the atmosphere and contribute to the auroral display over a range of altitudes. Other types of auroras have been observed from space; for example, "poleward arcs" stretching sunward across the polar cap, the related "theta aurora", and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as pulsating aurora, "black aurora" and their rarer companion "anti-black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) is observed around the two polar cusps, the field lines separating the ones that close through Earth from those that are swept into the tail and close remotely.
Altitude
Starting in 1911, Carl Størmer and his colleagues used cameras to triangulate more than 12,000 auroras. They found no auroras below and only 6.5% above , with a maximum in the height distribution around .
Forms According to Clark (2007), there are five main forms that can be seen from the ground, from least to most visible:
A mild glow, near the horizon. These can be close to the limit of visibility, but can be distinguished from moonlit clouds because stars can be seen undiminished through the glow. Patches or surfaces that look like clouds. Arcs curve across the sky. Rays are light and dark stripes across arcs, reaching upwards by various amounts. Coronas cover much of the sky and diverge from one point on it.
Brekke (1994) also described some auroras as "curtains". The similarity to curtains is often enhanced by folds within the arcs. Arcs can fragment or break up into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are also known as discrete auroras, which are at times bright enough to read a newspaper at night.
These forms are consistent with auroras being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the shapes of the luminous parts of the atmosphere and a viewer's position.
Colours and wavelengths of auroral light
Red: At its highest altitudes, excited atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this colour visible only under more intense solar activity. The low number of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are the most often seen hues of red for the auroras. Green: At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to the high stability of the N2 molecule) plays a role here, as it can transfer energy by collision with an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease in concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to forbidden transitions of atomic oxygen, a slow mechanism responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading.
Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity. The molecular nitrogen transitions are much faster than the atomic oxygen ones. White/continuum: White auroral emission (often appearing mauve) has been observed in the colour spectrum of STEVE, and within the aurora in the oval and on the poleward edge . The white colour is due to the pseudo-continuum spectrum of these auroral forms, which contains emission in all colours throughout the optical range, and has been observed with various spectrograph instruments. Ultraviolet: Ultraviolet radiation from auroras (within the optical window but not visible to the human eye) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars, Jupiter, and Saturn. Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras. Yellow and pink are a mix of red and green or blue. Yellow and pink auroras are relatively rare and are associated with high solar activity. Other shades of red, as well as orange and gold, also may be seen on rare occasions. As red, green, and blue are linearly independent colours, additive synthesis could, in theory, produce most human-perceived colours, but the ones mentioned in this article comprise a virtually exhaustive list.
Changes with time