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Why the night sky glows green
Not reflections, not clouds: auroras are light made high above us when particles from space meet our air.
On a dark night at high latitudes, the sky can begin to move. Pale arcs brighten, rays ripple, curtains of green light fold and unfold. It looks like weather, but it is happening far above the weather — at the edge of space.
Particles meet air
NASA describes auroras as the visible result of space weather: an interplay of particles and magnetism between the Sun and Earth. When energetic particles from space collide with atoms and molecules in our atmosphere, those atoms and molecules give off light. That glow is the aurora.
The particles involved are mostly electrons, guided by Earth’s magnetic field, according to the American Meteorological Society’s glossary, which draws on NOAA’s Space Weather Prediction Center. That magnetic guidance is why auroras are seen mainly in a band around the polar regions known as the auroral oval.
Colours tell you the height
Different gases glow in different colours, and the mix of gases changes with height. That turns an aurora into a kind of layered map.
Oxygen produces the familiar green, roughly 100 to 200 kilometres up, according to NASA. Higher still, above roughly 200 kilometres, oxygen glows red. Nitrogen adds blue tones at similar heights to the green, and pink lower down, below roughly 100 kilometres.
Most auroras occur between about 100 and 250 kilometres above the ground — far above any cloud.
North and south
Auroras happen around both poles. In the Northern Hemisphere they are called aurora borealis, the northern lights. In the Southern Hemisphere they are aurora australis, the southern lights.
Their shapes have names too. The glossary lists quiet arcs, moving rays, patches and veils among the forms an aurora can take.
Why it matters
Auroras are a reminder that Earth does not end at the top of the clouds. NASA notes that Earth’s magnetic field protects us from harmful charged particles in space. On certain nights, the edge of that protection becomes visible. Few natural phenomena show the connection between the Sun and our planet so clearly.
