Aurora: How the Solar Wind and Earth’s Magnetic Field Hang Curtains of Light Over the Polar Sky

On a winter night, a curtain of green light ripples slowly across the northern sky. Almost everyone has seen it in a photograph, yet surprisingly few people know where this light comes from. The solar wind crosses about 150 million km from the Sun and stores energy in Earth’s magnetic field; when that energy is released, electrons are accelerated and strike oxygen and nitrogen in the upper atmosphere, and the light they produce is the aurora. An aurora is not simply “a color in the sky” but a phenomenon in which the Sun, Earth’s magnetic field and the atmosphere are linked in a single chain.

It All Begins With the Sun

The aurora starts at the Sun. The Sun’s outermost layer, the corona, is so hot, at about two million °C, that charged particles such as electrons and protons stream out from it in every direction without pause. This flow is the solar wind. Near Earth, the slow solar wind moves at about 300 to 500 km per second, and the fast solar wind at about 750 km per second.

From time to time the Sun produces far more violent events. In a coronal mass ejection (CME), a mass of plasma from the corona breaks away all at once, carrying its magnetic field with it. Measured in coronagraph images as projected onto the plane of the sky, CME speeds vary enormously, from 20 to 3,200 km per second. A CME aimed at Earth usually arrives in one to three days, but a very fast one can take less than a day. The CME of the 1859 Carrington Event is estimated to have reached Earth in about 17.6 hours. Both the fast solar wind blowing from coronal holes and CMEs disturb Earth’s magnetosphere, and that disturbance is where the aurora begins.

Earth’s Shield, and the Gap in It

Earth has a shield: its magnetic field. The magnetosphere, the region Earth’s magnetic field carves out in space, deflects most of the solar wind around the planet. But much depends on the direction of the magnetic field the solar wind carries with it, the interplanetary magnetic field (IMF). When the IMF points northward, it cannot connect with Earth’s field on the dayside, and the solar wind flows past the magnetosphere. When the IMF points southward, the two fields break and reconnect on the dayside in a process called magnetic reconnection, and the solar wind drags the newly connected field lines over to the nightside. Field lines pile up in the long, stretched-out tail on the night side, the magnetotail, and energy is stored there under tension.

When the accumulated field lines become unstable, reconnection happens again in the magnetotail. Part of the plasma in the tail is then pushed toward Earth, and that motion feeds strong outbursts of aurora, mainly around midnight. This brief disturbance is called an auroral substorm, and to the eye it appears as auroral bands that suddenly brighten and move rapidly. It is distinct from a geomagnetic storm, which lasts for days. The particles that pour into the atmosphere here are not so much solar wind particles themselves as electrons and protons of the magnetospheric plasma, accelerated by the energy the solar wind has supplied.

The Moment It Becomes Light

Electrons pouring into the polar upper atmosphere collide with atoms and molecules of oxygen and nitrogen and give them extra energy. The excited particles then return to a stable state and release that energy as light. Which particle emits light, and at what height, determines the color of the aurora.

The most common color, green, is light at 557.7 nm emitted by oxygen atoms. The excited state of oxygen that produces this light lasts on average about 0.7 seconds before it emits light and returns to a stable state (its radiative lifetime). Because it comes from heights where oxygen atoms are relatively plentiful, and because the human eye is especially sensitive to green, green becomes the aurora’s “basic color.” Below an altitude of about 100 km, atomic oxygen drops off sharply, so the lower edge of the curtain ends crisply, as if cut with a knife. A study that analyzed all-sky camera records from northern Finland and Sweden over seven winters, from 2000 to 2007, found that green light was brightest at a mean altitude of about 114.8 km, and that blue light (427.8 nm) from molecular nitrogen ions usually peaked at a similar height.

Higher up than the green, red appears. Red (630.0 nm) is also emitted by oxygen atoms, but the average lifetime of the excited state that produces it is about 107 seconds, roughly 150 times that of green. If the atom collides with another particle in the meantime, it loses its energy and cannot emit light, a process called collisional quenching. So red survives only in the upper part of the curtain, where the air is extremely thin and collisions are rare. There are few oxygen atoms up there to begin with, and the human eye is less sensitive to this wavelength, so red stands out only when solar activity is strong. When red and green mix, the aurora can also look pink or yellow.

Unlike oxygen, nitrogen emits light quickly. Nitrogen molecules and molecular nitrogen ions glow at many wavelengths toward the red and the blue, and the most prominent of these is the 427.8 nm blue of molecular nitrogen ions. This blue and purple light commonly shows up at the lower edge of the curtain when solar activity is very strong.

Curtains and Coronas: What the Shapes Tell Us

Charged particles come down by spiraling around magnetic field lines. The shape of an aurora is therefore set by the shape of the glowing atmosphere together with the position of the person watching. Auroras seen from the ground are often divided into five forms: a faint glow near the horizon, patches that look like clouds, arcs that cross the sky, rays rising above the arcs, and coronas that cover most of the sky. In the evening an arc may stretch from horizon to horizon, and as midnight approaches it often begins to twist and sway.

Auroras usually form about 80 to 500 km above the ground. Starting in 1911, Carl Størmer of Norway and his colleagues used cameras to triangulate the heights of more than 12,000 auroras: they found none below 70 km, only 6.5% above 150 km, and the most at a height of about 100 km. When you look up from directly beneath rays lined up along the magnetic field lines, perspective makes them appear to spread out in all directions from a single point in the sky. This is the corona aurora.

Where and When You Can See It

Auroras are not visible just anywhere. The latitude that matters for auroras is geomagnetic latitude, measured not from the geographic poles but from the poles of Earth’s magnetic field (the geomagnetic poles). Auroras are seen most often in the auroral zone, a band about 6° (about 660 km) wide centered on about 67° geomagnetic latitude north and south. The region where auroras are glowing at any given moment is a ring around each geomagnetic pole, the auroral oval. Pushed by the solar wind, the oval is not centered on the pole but shifted toward the night side, so its arcs reach farthest toward the equator around magnetic midnight. According to the Royal Belgian Institute for Space Aeronomy, the oval is usually about 10° of latitude wide, and its equatorward edge moves between about 75° and 65° geomagnetic latitude depending on magnetic activity. In the Northern Hemisphere it passes over Alaska, Canada and Scandinavia; in the Southern Hemisphere it lies mostly over Antarctica.

Because the southern lights (aurora australis) and the northern lights (aurora borealis) form at the two ends of the same magnetic field lines, they often appear as near mirror images of each other, and when the aurora in the north changes, the one in the south changes almost at the same time. Such paired auroras are called conjugate auroras, and they tend to occur around the equinoxes, when there is little difference in how the north and south magnetic poles are tilted relative to the Sun.

According to the Space Weather Prediction Center of the US National Oceanic and Atmospheric Administration (NOAA), the best places to see the aurora are the high-latitude regions between roughly 60 and 75 degrees of latitude, where the auroral oval passes, and there the aurora can be seen on more than half of the nights in a year. Of course, there has to be enough activity, and the sky has to be dark and free of clouds.

Auroral activity follows the rhythm of the Sun. Solar activity rises and falls in a cycle of about 11 years, and large geomagnetic storms are most common at the peak of the sunspot cycle or in the three years after it. The size of a geomagnetic disturbance is expressed by the Kp index, from 0 to 9; the aurora’s position and brightness are closely tied to Kp, and when Kp is high (7 to 9) the aurora is bright and the oval moves to lower latitudes. On NOAA’s geomagnetic storm scale, at Kp 5 (G1) the aurora is commonly visible at high latitudes such as northern Michigan and Maine; it has been seen as low as New York and Idaho at Kp 6 (G2, typically 55° geomagnetic latitude), Illinois and Oregon at Kp 7 (G3, 50°), Alabama and northern California at Kp 8 (G4, 45°), and Florida and southern Texas at Kp 9 (G5, typically 40° geomagnetic latitude). In a typical 11-year cycle there are about two to four storms that reach Kp 9.

May 2024 was one of those times. From May 3 to 9, strong solar flares erupted one after another and at least seven CMEs headed toward Earth; they arrived in succession from May 10 and set off a G5 (Kp 9) geomagnetic storm, the strongest since 2003. NASA judged that the storm may have produced one of the strongest auroral displays on record in the past 500 years, and the aurora was observed at latitudes as low as Ferdows, Iran. The corona photograph above was also taken that night in Iowa.

A Light People Named and Recorded

The word “aurora” comes from Aurora, the Roman goddess of the dawn, who crossed the sky from east to west announcing the coming of the Sun; “borealis” comes from Boreas, god of the north wind, and “australis,” for the southern lights, from Auster, god of the south wind. Galileo Galilei used the term “aurora borealis” in his writings on the aurora in 1619, and Pierre Gassendi, who had read Galileo’s work, used it in 1649 when he described an aurora seen all over France in 1621.

Humanity’s records of the aurora are remarkably old. China’s Bamboo Annals contain an entry dated to 977 or 957 BC that is counted as a candidate for the earliest record of an aurora. Among records with a date clear down to the day, there are the astronomical diaries of Babylonia. A clay tablet for the night of March 12–13, 567 BC, reads “red glow flared up in the west,” followed by a duration of “2 danna” (one danna, or double-hour, is about two hours). Researchers interpret this as an aurora, pointing out that no other light source could give off red light for about four hours and that the land around Babylon is a flat plain without mountains. A 2016 study of the astronomical diaries from 652 BC to 61 BC found nine aurora-like records and considered five of them likely candidates.

The Roman writer Seneca left an anecdote that, under the emperor Tiberius, an aurora blazed so red over the port city of Ostia that an army unit stationed nearby for fire duty rushed off to put out the fire. In Scandinavia, the first text in which the “northern lights” (norðrljós) appear is the Norwegian King’s Mirror (Konungs Skuggsjá), from around 1230. Its author, who had heard of these lights from people returning from Greenland, offered three explanations: a vast fire surrounding the ocean, sunlight reaching around to the night side of the world, and glaciers that store energy and then glow. In the Southern Hemisphere, the Māori of New Zealand called the southern lights “great torches in the sky” (Tahunui-a-rangi), and a number of Aboriginal Australian traditions linked the red southern lights hanging low on the horizon with fire.

In more recent history, the most dramatic event is the Carrington Event of 1859. Just before noon on September 1, 1859, the English astronomers Richard Carrington and Richard Hodgson each independently observed an intense flash of light on the surface of the Sun; this was the first recorded observation of a solar flare. During the geomagnetic storm that followed on September 1 and 2, the aurora was seen in south-central Mexico, Cuba, Hawaii and southern Japan, and even in Colombia, close to the equator. The rapid changes in the magnetic field caused by the geomagnetic storm induced currents in the ground and in long wires (geomagnetically induced currents), knocking out telegraph networks in Europe and North America, and some operators even received electric shocks. Between Boston and Portland in the United States, there is a record of operators disconnecting their batteries and sending messages for about two hours using only these induced currents.

One Connected Picture

The solar wind streaming from the Sun, Earth’s magnetic field that receives and stores its energy and then lets it go, and finally the oxygen and nitrogen of the upper atmosphere that give off the light: the aurora is a single connected picture that these three paint together. That a star 150 million km away and the air we breathe are joined by invisible magnetic field lines to become a sweep of light quietly shows how finely the created world fits together. The next time you see a photograph of the aurora, it is worth remembering that every streak of green and red holds the long journey from the Sun to our atmosphere.

References

• Aurora – Wikipedia
• Auroras – NASA Science
• Aurora Tutorial – NOAA Space Weather Prediction Center
• Aurora – NOAA Space Weather Prediction Center
• NOAA Space Weather Scales – NOAA Space Weather Prediction Center
• Aurora – NOAA Science On a Sphere
• Magnetic Reconnection – ESA Science (Cluster)
• Whiter et al., The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora – Annales Geophysicae (2023)
• Auroral ovals at the two geomagnetic poles – Royal Belgian Institute for Space Aeronomy (BIRA-IASB)
• Solar wind – Wikipedia
• Coronal mass ejection – Wikipedia
• How NASA Tracked the Most Intense Solar Storm in Decades – NASA Science
• Hayakawa et al., Earliest datable records of aurora-like phenomena in the astronomical diaries from Babylonia – Earth, Planets and Space (2016)
• Substorm – Wikipedia
• Carrington Event – Wikipedia