The northern lights are caused by energy travelling from the Sun, interacting with Earth’s magnetic field, and finally being released as visible light in the upper atmosphere. The process starts roughly 150 million km away: the Sun releases a continuous flow of charged particles and magnetic field known as the solar wind, and when conditions are favourable, energy from that flow enters Earth’s magnetic environment and accelerates electrons toward the polar atmosphere. Those electrons collide with oxygen and nitrogen high above Earth, the gases absorb the energy and release it as light, and that light is the aurora borealis.
Five Connected Stages
The chain runs: the Sun releases the solar wind; that wind carries particles and magnetic field toward Earth; energy enters Earth’s magnetosphere; electrons accelerate along magnetic-field lines toward the polar atmosphere; and collisions with oxygen and nitrogen produce visible light. No single stage creates the aurora alone — it’s the whole Sun–Earth system working together.
Where the Energy Comes From
The Sun is an active star of extremely hot plasma, its surface and atmosphere shaped by powerful, constantly changing magnetic fields. It continuously releases charged particles — mainly electrons and protons — as the solar wind, which also carries part of the Sun’s magnetic field outward as the interplanetary magnetic field. This flow is never uniform: its speed, density, temperature and magnetic orientation are always shifting, and those variations determine how strongly it can interact with Earth.
Some activity comes from fast solar wind streaming out of coronal holes, regions where the Sun’s magnetic field opens more freely into space — as the Sun rotates, a long-lived coronal hole can repeatedly aim fast wind at Earth, sometimes driving several days of enhanced activity, though fast wind alone doesn’t guarantee northern lights; density, field strength and especially magnetic direction all matter too. Larger eruptions called coronal mass ejections (CMEs) launch a vast cloud of magnetised plasma that can reach Earth in roughly one to three days if Earth-directed, and a strong one can compress the magnetosphere and introduce powerful fields capable of driving a major, widespread event — but not every CME reaches Earth, and not every Earth-directed one produces strong aurora, since the outcome depends heavily on the magnetic orientation of what actually arrives.
Flares and CMEs are often confused but aren’t the same: a flare is a sudden release of electromagnetic radiation reaching Earth in about eight minutes, while a CME is a physical eruption of plasma and field that moves far more slowly. They can occur together, but one doesn’t imply the other — for aurora watchers, the real question is whether magnetised material or disturbed solar wind actually reaches Earth and interacts effectively with the magnetosphere.
Earth’s Magnetosphere and Why Bz Matters
Earth is surrounded by a magnetic field generated mainly by movement in its liquid outer core, and the region dominated by that field — the magnetosphere — deflects much of the incoming solar wind, compressed on the Sun-facing side and stretched into a long tail on the night side. It’s often called a protective shield, but it isn’t a sealed barrier; energy and particles can enter, circulate and become stored within it, and its shape and behaviour change continually with the solar wind.
One of the most useful short-term forecasting measurements is Bz, the north-south orientation of the interplanetary magnetic field near Earth. When Bz points south, opposite to Earth’s own dayside field, it favours magnetic reconnection — a process where magnetic fields with opposing directions meet, reorganise, and convert magnetic energy into particle motion, heat and current, letting solar-wind energy transfer more effectively into the magnetosphere. Sustained southward Bz lets the system store increasing energy and can drive stronger auroral activity; a strongly northward Bz generally makes that connection far less efficient, so activity can stay limited even when the solar wind itself is fast. Bz shouldn’t be read alone — solar-wind speed, field strength, density and how long favourable conditions last all matter too.
Reconnection on the dayside lets energy in; that energy is then transported into the magnetic tail behind Earth, where further reconnection can release it, accelerating particles and driving currents through the magnetosphere and upper atmosphere. The aurora is one visible result of this much larger energy transfer.
From Electrons to Light
Electrons accelerated within Earth’s magnetic environment are guided along field lines toward the polar atmosphere, where they collide with oxygen and nitrogen roughly 90 km to several hundred km up. Each collision pushes the gas into a temporary excited state; as it returns to a lower-energy state, it releases the extra energy as light — the same basic principle as a neon sign, just operating across a region that can span hundreds or thousands of kilometres.
Colour depends mainly on which gas is hit, the altitude, the incoming particles’ energy, and the surrounding atmosphere’s density. Green — the most common colour — comes from atomic oxygen around 100–150 km, and the eye’s strong sensitivity to green makes it easier to detect than dimmer emissions; weak green aurora can still look grey or white to the naked eye since human colour vision is poor in low light. Red also comes from oxygen, but higher up (often above ~200 km), where the thinner atmosphere gives excited atoms time to emit the slower red wavelength — it’s often faint and can show up more clearly in a camera than to the eye, though during strong storms it can spread widely and reach much lower latitudes than the usual green oval. Blue and violet come mainly from nitrogen, typically lower in the display where more energetic particles penetrate deeper; purple or pink often appears along the lower edge of a bright green curtain where nitrogen and oxygen emissions overlap, and cameras tend to pick up these tones more strongly than the eye does in the dark.
Arcs, Curtains and Movement
Auroral shapes reflect the structure of Earth’s magnetic field and how energetic particles distribute across the atmosphere. A quiet display might begin as a smooth arc; as activity rises, it can brighten, fold and separate into narrow rays roughly aligned with field lines, and many parallel rays together create the look of a curtain. When an active curtain passes overhead, perspective can make those parallel structures appear to radiate from a central point — an auroral corona, similar to railway tracks appearing to converge in the distance rather than actually meeting.
The aurora moves because the flow of particles, energy and current through the magnetosphere keeps changing — a quiet arc can sit nearly still while an active display brightens, folds, breaks apart or crosses the sky within seconds. This isn’t wind pushing a luminous cloud; it’s a changing pattern of where and how strongly particles are entering the atmosphere.
The Auroral Oval and Iceland
The northern lights concentrate within a broad ring around Earth’s magnetic pole called the auroral oval (with a matching one around the southern magnetic pole, producing the aurora australis). The oval is centred on the magnetic poles, not the geographic ones, and its position and width shift constantly — narrow and concentrated during quiet conditions, expanding toward lower latitudes as more energy enters the system. Iceland frequently sits beneath or close to the normal oval, which is why aurora can be visible here without a severe storm or a very high Kp.
Northern lights don’t require a major CME or an extreme geomagnetic storm — the solar wind is always interacting with Earth’s magnetosphere, and fast coronal-hole streams can produce repeated activity, especially when Bz turns south. A CME can produce a more intense, widespread event, but plenty of good Icelandic displays happen during fairly moderate space-weather conditions, which is one reason fixating only on dramatic flare or CME headlines can mislead you; conditions measured close to Earth are usually more useful than events observed at the Sun days earlier.
The roughly 11-year solar cycle changes the odds rather than switching the aurora on and off: near solar maximum, more sunspots, flares and CMEs raise the probability of major storms, while near solar minimum those explosive events become rarer — but long-lived coronal holes can still drive fast streams and repeated activity even then. For Iceland, the solar cycle mainly shifts the likelihood of exceptionally strong, far-reaching displays, not the basic possibility of seeing aurora at all.
Darkness, Cloud, and Substorms
The physical process behind the aurora can be fully active while nothing is visible from the ground — the sky has to be dark enough (Icelandic summer nights stay too bright even during real activity) and clear enough, since the aurora sits far above ordinary weather but cloud can block it completely. A powerful display above solid cloud simply can’t be seen; a weaker one under clear sky can be a far better experience. This is why a proper aurora forecast has to combine space-weather conditions with darkness and cloud cover, not just report one number.
An auroral substorm — energy stored in the magnetic tail releasing relatively quickly — is one reason a quiet sky can turn active within minutes: a calm arc suddenly brightens, develops rays and expands, often lasting from several minutes to over an hour. Substorms can happen during a major geomagnetic storm, but also on their own during much more moderate conditions, which is exactly why a local auroral intensification doesn’t need extreme global numbers behind it.
The northern lights come from a genuine transfer of energy — from the Sun, through interplanetary space, into Earth’s magnetic field, and finally into the upper atmosphere. Green and red come mainly from oxygen; blue, violet and some pink from nitrogen; and the display’s shape and movement reflect changing currents, fields and particle flows within the magnetosphere. What you’re seeing overhead is the visible end of a chain of events that started at the Sun.