An aurora is the visible end of a chain that starts on the Sun, crosses a hundred and fifty million kilometres of space, is caught and redirected by Earth’s magnetic field, and finishes with individual atoms in the upper atmosphere emitting specific wavelengths of light.
The whole system is a demonstration that the space between the Sun and Earth is not empty.
The chain, step by step
The Sun leaks
The Sun’s outer atmosphere is hot enough that its plasma is not gravitationally bound. It streams outward continuously as the solar wind — a thin flow of protons and electrons moving at hundreds of kilometres per second, permeating the entire solar system.
The Sun also produces episodic events: coronal mass ejections, which are enormous eruptions of magnetised plasma. These are what drive the strongest auroral displays, and their arrival is what space weather forecasters watch for.
Earth deflects most of it
Earth generates a magnetic field in its liquid iron outer core. That field carves out a cavity in the solar wind — the magnetosphere — which deflects the majority of incoming particles around the planet, compressed on the sunward side and drawn into a long tail behind.
This shielding is not incidental. It is one of the reasons Earth retained its atmosphere while Mars, which lost its global magnetic field early, did not.
Some particles get in anyway
The magnetosphere is not sealed. When the solar wind’s magnetic field is oriented opposite to Earth’s, the two can reconnect, opening pathways for particles to enter and be stored in the magnetotail.
Reconnection events in the tail then accelerate those particles back along field lines toward Earth. Because magnetic field lines converge at the poles, the particles are funnelled into two rings around the magnetic poles — the auroral ovals.
Atoms light up
At altitudes between roughly 100 and 400 kilometres, the incoming electrons collide with atmospheric atoms and molecules, kicking their electrons into higher energy states. When those electrons drop back down, they emit photons at wavelengths determined entirely by the atom involved.
- Green, the dominant colour, comes from atomic oxygen at around 100 to 250 kilometres.
- Red comes from atomic oxygen higher up, above roughly 250 kilometres, where collisions are rare enough to allow a slow transition to complete.
- Blue and purple come from ionised nitrogen, usually at lower altitudes and typically visible only in stronger displays.
The colours are not a lighting effect. They are the emission spectra of specific atoms, read directly off the sky.
Why the shapes move
Auroras are rarely static. They form arcs, curtains, rays, and occasionally fill the whole sky in a rapid brightening called a substorm.
The vertical striping in curtains traces magnetic field lines — the particles are following them down, so the visible structure maps the field’s geometry. The rapid motion during substorms reflects the sudden release of energy stored in the stretched magnetotail.
The two hemispheres light up more or less simultaneously and often in near-mirror-image patterns, since the same field lines connect north and south. The southern lights are less frequently observed simply because there is less land under the southern auroral oval.
Seeing them
Auroras are best hunted with a few practical rules.
Location matters most. The auroral oval typically sits over northern Scandinavia, Iceland, northern Canada, Alaska, and Siberia. During strong geomagnetic storms it expands toward the equator, occasionally reaching mid-latitudes — which is when auroras make the news.
Timing follows the solar cycle. Solar activity rises and falls over roughly eleven years, and displays are more frequent and more intense near solar maximum. Within a night, the hours around local magnetic midnight tend to be most active.
Dark and clear are prerequisites. Aurora hunting has the same enemies as any night-sky activity: cloud, moonlight, and streetlights.
Forecasts are genuinely useful. The Kp index gives a rough measure of geomagnetic disturbance on a scale of zero to nine, and satellites at the L1 point upstream of Earth provide roughly thirty to sixty minutes of warning as the solar wind arrives.
One expectation to adjust: cameras are more sensitive to faint aurora than the eye is, particularly to red. A display that photographs vividly may look pale grey-green in person. That is not a failure of the aurora; it is a property of human night vision, which is poor at colour.
The part that is not decorative
The same solar events that cause auroras can induce currents in long conductors on the ground — power grids, pipelines, undersea cables. Severe geomagnetic storms have caused regional blackouts and have damaged transformers.
They also degrade radio communication, disrupt satellite navigation, increase drag on spacecraft in low orbits, and pose a radiation risk to astronauts and to passengers on polar flight routes.
Aurora forecasting exists mainly for that reason. The lights are the visible symptom of a system that industrial infrastructure has become quite sensitive to.