The aurora — the northern lights (aurora borealis) and southern lights (aurora australis) — is one of the few genuinely spectacular sky phenomena you can see with the naked eye. For centuries it was tied to omens and dread; only over the last ~150 years did we figure out what actually causes it, and parts of it are still being researched today (and observed on other planets too).
This is a high-level nutshell based on the lecture by Paul Koenraad and Anne-Marie Koenraad at Halley Observatory — see 2026-06-28 Lezing over het Poollicht bij Halley.
In a nutshell — how it works
- The Sun acts up. Sunspots, solar flares and coronal mass ejections fling charged particles (the solar wind) out into the solar system. After an active region on the Sun, it takes roughly 3 days for the effects to reach us.
- Earth’s magnetic field catches it. The solar wind pushes on Earth’s magnetic field, stretching the field lines on the night side into a long “tail”.
- Magnetic reconnection. Far out in that tail, stretched magnetic field lines snap and reconnect — like a stretched rubber band suddenly released — and rebound back toward Earth. This reconnection, not the rotation axis, is the main driver of the brightest aurora.
- Electrons surf a wave. The rebound launches Alfvén waves travelling back along the field lines. Electrons “surf” on the electric field of those waves and are accelerated — the energy-transfer mechanism first reasoned out by Lev Landau in 1946 (Landau damping).
- The light. These electrons slam into oxygen and nitrogen high in the atmosphere (the thermosphere, ~100–250 km up), making them glow. That glow is the aurora.
A neat consequence: the aurora forms a ring around the magnetic pole, on the day/night boundary, and the two halves have different origins. The night-side aurora is brighter and more beautiful — not just because it’s dark, but because those particles took the longer route out past Earth and back via reconnection, arriving with more energy than if they’d come straight from the Sun.
The 20,000 km/s confirmation
Landau worked this out on paper in 1946, but it was only confirmed experimentally in 2021. Physicists (University of Iowa, using UCLA’s Large Plasma Device) showed definitively that Alfvén waves can accelerate electrons up to about 20,000 km/s (~45 million mph) — fast enough to produce the brightest auroral arcs when they hit the upper atmosphere.
See the popular write-up: Physicists determine how auroras are created (University of Iowa, 2021), based on the paper Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves (Nature Communications).
The colours
The colour depends on what gets hit and how deep the electrons penetrate:
- Red (~250 km) — atomic oxygen, highest up.
- Green (~100–250 km) — atomic oxygen lower down; the classic, most common colour.
- Purple / blue (lowest) — molecular nitrogen. Rarer, because only the most energetic electrons reach that deep.
Shape
The aurora appears mostly as shimmering curtains — essentially 2D sheets, not 3D blobs. Why they take this flat, sheet-like form is one of the things we still don’t fully understand. Depending on your viewing angle you see stripes (from below) or bands (from the side). Early in the evening they look curtain-like; toward morning their character changes and they fade.