INSIGHT: Northern Lights — The Science Behind the Magic of the Sky

Oxygen, nitrogen, altitude: the spectrum of the aurora

When accelerated electrons plunge into the atmosphere along magnetic field lines, they collide with oxygen and nitrogen atoms and molecules present at altitudes between 80 and 400 kilometers. These collisions transfer energy to the atmospheric atoms, bringing them into what is known as an excited state—an unstable state in which the atoms’ electrons have jumped to higher energy levels. To return to their stable state, these atoms release the excess energy in the form of photons —particles of light. This is exactly the same principle at work in a neon sign or a fluorescent lamp: electricity excites the atoms of a gas, which then release this energy as colored light.

The color emitted depends on which atom or molecule is excited, and at what altitude the collision occurs. NASA’s data is precise:atomic oxygen between 100 and 250 kilometers in altitude emits a bright green —the most common color of the auroras. The same atomic oxygen above 200 kilometers emits a deep red —characteristic of the large red arcs sometimes visible at low latitudes during intense storms.Molecular nitrogen, at lower altitudes, emits blue or violet. The combination of these colors produces the pinks, mauves, and whites seen during particularly active auroras.

Why the auroras move and undulate

The characteristic movement of the auroras—those undulating curtains, twisting arcs, and pulsating columns—is a direct reflection of the dynamic variations in Earth’s magnetic field and fluctuations in the particle flux. When magnetic field lines vibrate under the influence of plasma waves—known as Alfvén waves —the electrons traveling along them accelerate and decelerate in rhythm, causing the auroras to brighten and dim in sync with these oscillations. Studies from NASA’s THEMIS mission have shown that during certain events, the aurora borealis moves in harmony with 6-minute oscillations of the magnetic field lines near Earth.

The various forms of the auroras—arcs, curtains, crowns, and spots—correspond to different configurations of the magnetic field and different entry points for particles into the atmosphere. A stable auroral arc corresponds to a zone of steady particle precipitation along a specific field line. Bands and curtains appear during more intense magnetic disturbances. The auroral crown —where the curtains seem to converge toward a point directly above the observer—corresponds to the precise zone above the magnetic pole, where the field lines enter the atmosphere vertically.

I find that auroras are one of the few things in nature that are both more beautiful and more understandable once you understand how they work. Knowing that these green curtains dance to the rhythm of the Earth’s magnetic field vibrations, that each pulse is an Alfvén wave—it doesn’t take away from the wonder. It enriches it.

The 11-year cycle that governs the auroras

Solar activity follows an approximately 11-year cycle between a solar minimum —a period of calm with few sunspots and CMEs—and a solar maximum, a period of intense activity with numerous eruptions. During solar maxima, the auroras are more frequent, more intense, and visible at significantly lower latitudes. Earth is currently nearing the peak of Solar Cycle 25, which explains the spectacular northern lights observed in 2024 and 2025 as far south as France, the northern United States, Germany, and Japan—regions that do not normally see them.

During a major geomagnetic storm,the auroral oval can expand enough for auroras to be visible from northern France, Brittany, or even further south. These rare events correspond to a geomagnetic Kp index greater than 7 or 8 on a scale of 9. Kp-9, the maximum level, is associated with superstorms like those of 1859 (the Carrington Event)—a solar flare so powerful that it caused telegraphs to operate on their own and made the auroras visible as far away as Central America and Italy.

The Carrington Event: What the Northern Lights Reveal About Our Vulnerability

The Carrington Event of 1859 is the largest geomagnetic storm recorded in modern history. If an event of this magnitude were to occur today, the consequences would be far more severe: overloading and potential destruction of high-voltage electrical transformers, disruption of satellites in orbit, deactivation of GPS networks, and interruption of radio communications. Estimates of the economic damage from such a storm today range from 600 billion to several trillion dollars. The Northern Lights are therefore not only beautiful—they are also a visible indicator of a real risk to our technological infrastructure.

That is why agencies such as NOAA in the United States and the European Space Weather Center monitor solar activity, coronal mass ejections, and the geomagnetic index in real time. Aurora forecasts are not just for space tourism—they allow operators of power grids, satellites, and communications networks to prepare for potential disruptions. The colorful sky you’re admiring from a lake in northern Iceland is also a warning signal for the engineers tasked with protecting infrastructure.

This duality of the auroras always strikes me: on the one hand, the most magical thing you can see with your own eyes in the night sky; on the other, the visible manifestation of a force that, if powerful enough, could black out our cities and paralyze our satellites. Beauty and threat in the same green curtain.

The Northern Lights as a Natural Laboratory

The Northern Lights are not just a spectacle. They are a natural laboratory for plasma physics accessible from the ground. By studying their shapes, colors, and movements, scientists map the structure of Earth’s magnetic field, measure particle fluxes in the magnetosphere, and test fundamental theories on magnetic reconnection and the dynamics of space plasmas. Satellites such as ESA’s THEMIS and CLUSTER missions simultaneously observe the auroras from space and from the polar regions, making it possible to link what is happening in the magnetosphere thousands of kilometers above the Earth’s surface to what we see from the ground.

Take a look tonight if you can

The next time you find yourself under a dark sky at a high enough latitude on a night of solar activity, look toward the north. If greenish or reddish glows are dancing above the horizon, remember what you’re really looking at: an electron that left the Sun a few days ago, guided by Earth’s magnetic field up into the upper atmosphere, where it transfers its energy to an oxygen atom at an altitude of 150 kilometers, which in turn releases a green photon that travels through the entire atmosphere to strike your retina. A journey of 150 million kilometers for this moment of beauty. It’s not magic. It’s better than that.

By Maxime Marquette, columnist