(Every day until December 25, I’m posting a science-related image or video and description.)

Day 16

I’ve never seen an aurora except in pictures, though I really hope to before I kick the cosmic bucket.[1] While they are occasionally visible in the more southern regions where I’ve lived, they’re more common the farther north (or if you’re in the southern hemisphere) the farther south you go. From photos alone, they are awe-inspiring sights: shimmering ripples of green, blue, or red hanging in the sky.

While auroras are beautiful, they are actually the result of one our planet’s natural defenses against the Sun. Not that Earth is conscious of the needs of life, but a lot of the Sun’s output is very damaging. X-rays and ultraviolet radiation are bad for living tissues, and the solar wind – charged particles like electrons and protons accelerated by the Sun’s magnetic field to high speeds – is similarly unconducive to health. However, Earth’s atmosphere blocks a lot of the harmful light, while its magnetic field deflects and steers the solar wind.

Auroras — the Northern and Southern Lights — are the result of the shape of the magnetic field. Earth is like a bar magnet, with the ends of the bar close to the North and South Poles. That’s why you can use a magnetic compass to find north — it’s roughly the same place where magnetic north is.[2] The magnetic field exerts forces on the moving solar wind particles, channeling them along paths that concentrate them in vast regions outside Earth’s atmosphere called the Van Allen radiation belts.

The magnetic field doesn’t prevent all of these particles from reaching the atmosphere, but they are channeled preferentially toward the arctic and antarctic regions. When they strike molecules in the atmosphere, they can ionize them — stripping electrons off — or cause electronic transitions, moving the electrons between energy states. The recombination of the electron to the molecule and the return to ground state both produce light: green and red for oxygen, blue and red for nitrogen. The image above is predominantly green, but there are many colorful displays depending on the energy of the solar wind particles. (Auroras are also prominent on the giant planets.)

So if you’re fortunate enough to see auroras, think of their beauty, but also of how they’re a visible sign of the invisible magnetic shield protecting the planet. Ours is a wonderful universe.

Notes

1. Shuffle off the mortal magnetic coil? Increase the earthly entropy? Collapse my personal wavefunction?
2. Confusingly, the poles of magnets are also labeled north and south, but these are opposite to those on Earth. Our planet’s geographical north pole corresponds to the south pole of Earth’s magnetic field, and vice versa.