The Northern Lights
The Sun throws particles at us, Earth's magnetic field catches them, and the air glows in colours set by altitude.
Why the sky glows green
Junior level — plain language, no maths
The Sun is not just shining at us, it is throwing things at us. A thin wind of electrically charged particles streams off it constantly at hundreds of kilometres per second, and every so often a storm on the Sun's surface hurls out a much denser cloud of them.
Those particles never reach most of us, because the Earth has an invisible shield: its magnetic field. It wraps the planet like an enormous force field, deflecting the stream around us the way a rock splits a river. But the shield has two weak spots - the poles - where the field lines dive down into the ground instead of arching over us.
Particles that get funnelled down there slam into the air about a hundred kilometres up. They hit atoms of oxygen and nitrogen and knock their electrons out of place, and when those electrons drop back, the atom gives the energy back as light. That is the aurora: not reflected light, not fire, but air being hit hard enough to glow.
The colour is a height measurement. Oxygen struck around 100 to 200 km up glows the familiar green. Much higher, above 250 km, the same oxygen glows deep red. Nitrogen lower down adds blue and violet along the bottom edge of the curtains. So the ribbon of colour in the sky is telling you exactly how deep into our atmosphere the particles managed to get.
Things worth knowing
- Auroras happen at the poles because that is where Earth magnetic field lines dive into the ground, funnelling particles down with them.
- Green comes from oxygen around 100-200 km up, red from oxygen above 250 km, blue and violet from nitrogen below 100 km.
- Earth is not special: Jupiter and Saturn have auroras too, and Jupiter is so magnetic that its aurora never switches off.
Solar wind, reconnection and the auroral oval
Student level — the core equations
The solar wind carries the Sun's magnetic field with it, and the direction that field points when it arrives matters more than how fast it is going. This is the interplanetary magnetic field, and the component that counts is \(B_z\), the north-south one.
When \(B_z\) points north, it lines up with Earth's own field at the nose of the magnetosphere and the two simply press against each other: the shield holds and very little energy gets in. When \(B_z\) points south, the two fields are antiparallel and they reconnect - they break and splice together, opening the shield. Solar wind energy pours in, loads the magnetotail behind us, and eventually the tail snaps back and fires particles down the field lines into the polar atmosphere.
The result is not a blob over the pole but a ring: the auroral oval, centred on the magnetic pole and typically sitting near 67° magnetic latitude. As the storm strengthens the oval expands towards the equator, which is why a strong event is seen from Scotland or Michigan and a severe one from Rome or Texas. Geomagnetic activity is graded on the \(K_p\) index, 0 to 9, and each step down in latitude costs roughly one \(K_p\) unit.
The light itself comes from forbidden atomic transitions. The green 557.7 nm line of atomic oxygen takes about 0.7 s to fire, and the red 630.0 nm line takes over 100 s - which is why red only appears high up, where the air is so thin the excited atom is not knocked out of its state by a collision before it can emit.
Key Formulas
| Green oxygen line | \(\lambda = 557.7\ \text{nm}\) | lifetime ~0.7 s, 100-200 km |
|---|---|---|
| Red oxygen line | \(\lambda = 630.0\ \text{nm}\) | lifetime >100 s, above 250 km |
| Oval latitude | \(\Lambda \approx 67^\circ - 2.5\,K_p\) | rough, equatorward edge |
Things worth knowing
- A southward interplanetary field is the trigger: antiparallel fields reconnect at the dayside magnetopause and open Earth shield.
- The red 630 nm oxygen line has a lifetime over 100 s, so it only survives above ~250 km where collisions are rare - that is why red sits above green.
- Each unit of Kp pushes the auroral oval roughly 2-3 degrees of latitude further from the pole.
Reconnection, the substorm cycle and forbidden lines
Scholar level — full mathematical depth
The magnetosphere is an open system driven by the Dungey cycle. Dayside reconnection between a southward IMF and the geomagnetic field erodes the magnetopause, transporting open flux over the polar cap into the tail lobes. Loading continues until the near-Earth plasma sheet thins enough for tail reconnection to onset, at roughly \(20\,R_E\) downtail; the released energy accelerates electrons earthward along field lines, and their precipitation lights the substorm auroral bulge.
Coupling scales with the solar wind electric field, \(E_y = -v B_z\) for southward \(B_z\), which is why a modest 500 km/s stream with a strongly southward field outperforms a fast stream with a northward one. Coronal mass ejections deliver the strongest events because they carry both high speed and an ordered, often strongly southward, internal field; declining-phase coronal holes instead give recurrent high-speed streams that produce moderate storms on a 27-day rhythm as the Sun rotates.
The emission is diagnostic. The 557.7 nm and 630.0 nm oxygen lines are both forbidden transitions - \(^1S \to {}^1D\) and \(^1D \to {}^3P\) - with radiative lifetimes of 0.7 s and 110 s. Collisional quenching therefore sets an altitude floor for each: below about 250 km the \(^1D\) state is de-excited before it can radiate, so red is absent from low altitudes no matter how hard the atmosphere is hit. The N\(_2^+\) first negative bands at 427.8 nm, being allowed transitions, mark the deepest penetration and hence the hardest precipitating electrons.
The practical stakes are not aesthetic. The same currents that light the oval drive geomagnetically induced currents in long conductors: the March 1989 storm collapsed the Hydro-Québec grid in 92 seconds, and the 1859 Carrington event induced currents strong enough to set telegraph paper alight.
Key Formulas
| Solar wind coupling | \(E_y = -v_x B_z\) | the driver of the cycle |
|---|---|---|
| Forbidden transitions | \({}^1S \to {}^1D:\ 557.7\ \text{nm};\quad {}^1D \to {}^3P:\ 630.0\ \text{nm}\) | |
| Radiative lifetimes | \(\tau \approx 0.7\ \text{s},\ 110\ \text{s}\) | sets the altitude of each colour |
| Tail reconnection | \(x \approx -20\,R_E\) | |
Things worth knowing
- The Dungey cycle: dayside reconnection opens flux, the tail stores it, and tail reconnection at ~20 Earth radii releases it as a substorm.
- Both bright oxygen auroral lines are forbidden transitions - they are only possible because the upper atmosphere is thin enough to avoid collisional quenching.
- The March 1989 geomagnetic storm collapsed the Hydro-Quebec power grid in 92 seconds through geomagnetically induced currents.