Transcript:

So, a couple of my friends posted some pretty cool photos of the northern lights. And in most of those photos, the sky had a beautiful hue of green. And here's why the sky was green when those guys, my friends, went to see the northern lights.

So what happens is, the sun sends out high-energy particles called the solar wind. And most of the time, these high-energy particles are deflected by the Earth's magnetic field. They just get bounced off of it. But when the solar wind is higher intensity, these particles penetrate past the magnetic field and enter the atmosphere.

And when they enter the atmosphere, if they excite the oxygen molecule, you see a hue of green. If those high-energy particles excite a nitrogen molecule, you would see hues of blue and red. And Earth's atmosphere is predominantly oxygen and nitrogen — 20.9% oxygen, 79.1% nitrogen.

So most of the times, these northern lights have a hue of green or blue or red, because these are the excitation states of oxygen and nitrogen. Now you know.