Aurora

The aurora is light produced when electrons collide with the upper reaches of Earth's atmosphere, appearing in the Northern Hemisphere as the aurora borealis and in the Southern Hemisphere as the aurora australis. It occurs in an oval around the magnetic poles in both hemispheres, so that the southern lights and the northern lights are the two faces of a single phenomenon. The display appears as two ovals approximately centered on the magnetic poles. At polar latitudes the aurora appears on about half the nights of a given year.

Auroral light begins with the solar wind, the stream of charged particles emitted by the sun. As that wind flows outward it drags the Sun's magnetic field along with it, and the Sun's rotation stretches the field into a spiral shape. Energy from the solar wind is exchanged very efficiently into the space environment around Earth during a geomagnetic storm, a major disturbance of the magnetosphere. Such storms are created most effectively by sustained periods of high-speed solar wind together with a southward-directed solar wind magnetic field at the dayside of the magnetosphere. Less intense but longer-lasting driving comes from high-speed streams that plow into slower solar wind and form co-rotating interaction regions, which deposit more energy over a longer interval than coronal mass ejection storms.

A coronal mass ejection is a large expulsion of plasma and magnetic field from the Sun's corona that can carry billions of tons of material. These ejections travel outward at speeds ranging from below 250 km/s to nearly 3000 km/s, and the fastest ones directed at Earth can arrive in as little as 15 to 18 hours while slower ones take several days. The most explosive ejections begin when highly twisted magnetic flux ropes in the lower corona become too stressed and realign through a process called magnetic reconnection.

The particles that actually produce the glow are electrons and protons, and their precipitation injects millions of amps into the atmosphere. Collisions between these particles and oxygen and nitrogen atoms and molecules excite them to higher energy states, which release light on relaxation. The colour that results depends on which gas is struck and at what altitude the collision occurs.

Three ingredients are required for a planet to have an aurora: solar wind, a magnetic field, and an atmosphere. Earth's magnetosphere, the region of space dominated by the planet's own magnetic field, shields the planet from solar and cosmic particle radiation and from erosion of the atmosphere by the solar wind. Constant bombardment compresses the sun-facing side of that field, though the dayside still extends about six to 10 times the planet's radius, while the nightside stretches into an immense magnetotail that can measure hundreds of Earth radii, reaching far beyond the Moon's orbit at 60 Earth radii. The gases of the atmosphere give off light when struck, much as a neon sign glows. During a storm the westward ring current around Earth is tracked by the disturbance storm time index, while field-aligned currents connect to intense auroral electrojets that also produce large magnetic disturbances.

Aurora appears at a typical altitude range of about 80 to 500 km above Earth's surface. The best viewing latitudes lie between about 60 and 75 degrees north and south latitude. The northern lights form a band that is normally about 500 to 1,000 km wide, positioned on average over Yellowknife in the Northwest Territories and Grande rivière de la Baleine in Quebec.

The auroral oval's equatorward edge sits near 66 degrees magnetic latitude at Kp 0 and moves about 2 degrees equatorward per Kp step to roughly 48 degrees at Kp 9. During intense activity the oval may expand as far south as Miami, Florida, a reach recorded in August 1972 and beyond. The expansion toward the equator can reach over Tasmania.

The colour of an aurora depends on which gas is struck and where the collision occurs. Green is the most common colour, produced by oxygen at roughly 100 km altitude; green appears from about 100 to 200 km while red appears above 200 km. Red is caused by oxygen high in the atmosphere, green by oxygen in the middle layers, and purple by nitrogen lower down.

The green line at 557.7 nm (5577 Å) is emitted by atomic oxygen and is generally the brightest auroral emission. The red line at 630.0 nm (6300 Å) comes from atomic oxygen in its transition from the lowest excited electronic state to the ground state, is sometimes seen at the upper border of arcs, and has peak emission typically between 200 and 400 km. The blue-violet 427.8 nm (4278 Å) emission lies in the blue region of the visible spectrum and comes from the molecular nitrogen ion.

Great red auroras are rare because they require a perfect mix of heavenly conditions. Pure red forms much higher, from about 200 to 300 miles up, where oxygen remains and electrons produce red light as pure as a laser. Red auroras have to be 10 times as bright as green auroras for that color to be visible to the naked eye. Cameras and phones that can capture more light wavelengths than the human eye sometimes show red in aurora photos in which the photographer only saw greens.

Auroral forecasting is commonly expressed through the planetary K index, or Kp, a scale running from 0 to 9 that was introduced in 1939 by the German scientist Julius Bartels. At its lowest values the aurora stays far to the north and remains dim. As Kp rises through the range of 3 to 5 the aurora moves away from the poles and grows brighter, with more motion and more varied formations. At Kp 8 to 9 it is pushed toward the equator, very bright and active, with the extended oval overhead from the northern states of the USA. A Kp above 5 marks the threshold of a geomagnetic storm.

A 30-minute aurora forecast product is built on the OVATION model, which stands for Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting, an empirical model of auroral intensity developed at the Johns Hopkins University Applied Physics Laboratory. It projects the location and intensity of the aurora 30 to 90 minutes ahead. Forecast lead time is defined as the solar wind travel time from the L1 observation point to Earth. When input data are contaminated or unavailable, the product falls back to a current-Kp estimate with no lead time. NASA TIMED GUVI data were added so that Kp 8 and 9 values could be captured beyond the DMSP-only version, which is reliable to Kp 7.

NOAA's Geomagnetic Forecast projects activity for the next three UTC days in 3-hour intervals, updated daily at 22:05 UTC. Some aurora is always occurring somewhere on Earth, but the sky must be dark and at least partly clear for it to be visible, and a calm solar wind can leave it faint and confined to very high latitudes. Active displays become more diffuse and fragmented later in the night, so 9 p.m. is the higher-probability window over interior Alaska in winter. Auroras can be predicted, though not guaranteed, hours or days ahead of time from observations of the sun, satellite measurements of the solar wind and modelled changes in geomagnetic activity.

Digital camera sensors are sensitive enough to record auroras that the naked eye cannot see. On a clear night the chance of observing an aurora in Tasmania averages around 1% to 2% and is strongly correlated with the sunspot cycle. The earlier in the evening an auroral display is seen in the southern sky the more likely it is that the display will become more spectacular and more overhead around midnight.

George Siscoe assembled reasons to believe that the term aurora borealis was introduced by Galileo Galilei in 1619, though the phrase was already in use in 1621 by the French scientist and philosopher Pierre Gassendi. Elias Loomis of Yale University compiled in 1860 a map marking how many times in an average year auroras were observed at various locations. Kristian Birkeland conducted a magnetized terrella experiment inside a vacuum chamber, in which a beam of electrons was steered by the magnetic field toward the sphere's poles.

Detectors aboard a rocket launched into the aurora by Meredith, Gottlieb and Van Allen in 1954 produced the first observations of auroral electrons. A rocket experiment in 1959 identified the particles as electrons accelerated by 6000 volts.

Auroral forms of all types and colors were seen below 50 degrees latitude for about 24 hours on August 28-29 and about 42 hours on September 2-3; at its height it was a blood or deep crimson red so bright one could read a newspaper by it. Richard Carrington and a second observer independently became the first to observe a white light flare on 1 September 1859 while watching a large sunspot group. Ionospheric currents ran so strong that magnetometers frequently went off scale and telegraph systems became inoperable.

A barrage of large solar flares and coronal mass ejections in the first full week of May sent clouds of charged particles toward Earth, creating the strongest solar storm to reach the planet in two decades. Eight X-class flares occurred between 7 and 11 May, with the strongest peaking at X5.8, alongside at least seven coronal mass ejections. The ejections bunched in waves that reached Earth from May 10 onward, creating a long-lasting storm not seen since 2003 and rated G5 at the top of the geomagnetic storm scale. By the disturbance storm time index the event was similar in strength to the historic storms of 1958 and 2003, with auroras reported as low as 26 degrees magnetic latitude. NOAA's Space Weather Prediction Center issued pre-storm alerts to power grid and commercial satellite operators, and NASA's ICESat-2 entered safe mode likely because of increased drag.

The 1989 geomagnetic storm stands as the most severe on record since an event in 1921, causing power blackouts in Canada and the US along with radio communication blackouts and satellite anomalies. A fast, dense gust of solar wind passing Earth can temporarily compress the planet's magnetic field, and the resulting change in field strength can burn out power grid transformer stations and cause blackouts.

Astronomers turned NASA's Hubble Space Telescope toward the poles of Jupiter, the largest planet in the solar system, and photographed its auroras in far-ultraviolet light as NASA's Juno spacecraft approached and entered orbit. The Hubble program was designed to establish how Jupiter's auroras answer to shifting conditions in the solar wind, the outflowing solar wind. Such displays arise when charged particles are accelerated along a planet's magnetic field to high energies and strike the atmosphere near the magnetic poles, setting it aglow. The scale of the Jovian case is set by its magnetosphere, which is 20,000 times stronger than Earth's.

The northern lights have held great cultural importance for many peoples for thousands of years, and to many such as the Cree and Inuit they symbolize spirits dancing across the sky. For many Indigenous peoples they represent the spirits of ancestors and a connection to the spirit world, and they are treated with great respect and reverence.

Nicholas Flowers notes that the behaviour of the northern lights can help predict weather, since still lights suggest fair weather and light winds the following day while quickly dipping lights suggest less ideal conditions.

• 1619
  Term introduced
  George Siscoe has given reasons to believe the term aurora borealis was introduced by Galileo Galilei. ^{17The Polar Aurora · NASA Goddard Space Flight Center (Polar Wind education pages) ↗}
• 1621
  Term used
  The term was used by the French scientist and philosopher Pierre Gassendi. ^{17The Polar Aurora · NASA Goddard Space Flight Center (Polar Wind education pages) ↗}
• 1954
  First auroral electron observations
  Detectors aboard a rocket launched into the aurora by Meredith, Gottlieb and Van Allen made the first observations of auroral electrons. ^{17The Polar Aurora · NASA Goddard Space Flight Center (Polar Wind education pages) ↗}
• 1959
  Electrons identified
  A rocket experiment identified the particles as electrons accelerated by 6000 volts. ^{17The Polar Aurora · NASA Goddard Space Flight Center (Polar Wind education pages) ↗}