What causes the different colors of the aurora? An expert explains the electric rainbow

Final week, an enormous solar flare despatched a wave of energetic particles from the sun surging out by way of space. Over the weekend, the wave reached Earth, and folks world wide loved the sight of unusually vivid aurora in each hemispheres.

Whereas the aurora is often solely seen near the poles, this weekend it was noticed as far south as Hawaii within the northern hemisphere, and as far north as Mackay within the south.

This spectacular spike in auroral exercise seems to have ended, however don’t be concerned when you missed out. The sun is approaching the height of its 11-year sunspot cycle, and intervals of intense aurora are prone to return over the subsequent 12 months or so.

In case you noticed the aurora, or any of the pictures, you could be questioning what precisely was occurring. What makes the glow, and the completely different colours? The reply is all about atoms, how they get excited—and the way they chill out.

When electrons meet the environment

The auroras are brought on by charged subatomic particles (principally electrons) smashing into Earth’s environment. These are emitted from the sun on a regular basis, however there are extra throughout instances of higher solar exercise.

Most of our environment is protected against the inflow of charged particles by Earth’s magnetic discipline. However close to the poles, they will sneak in and wreak havoc.

Earth’s environment is about 20% oxygen and 80% nitrogen, with some hint quantities of different issues like water, carbon dioxide (0.04%) and argon.

When high-speed electrons smash into oxygen molecules within the upper atmosphere, they break up the oxygen molecules (O₂) into particular person atoms. Ultraviolet gentle from the sun does this too, and the oxygen atoms generated can react with O₂ molecules to supply ozone (O₃), the molecule that protects us from dangerous UV radiation.

However, within the case of the aurora, the oxygen atoms generated are in an excited state. This implies the atoms’ electrons are organized in an unstable manner that may “chill out” by giving off vitality within the type of gentle.

What makes the inexperienced gentle?

As you see in fireworks, atoms of various parts produce completely different colours of sunshine when they’re energized.

Copper atoms give a blue light, barium is inexperienced, and sodium atoms produce a yellow–orange colour that you could be even have seen in older road lamps. These emissions are “allowed” by the foundations of quantum mechanics, which suggests they occur in a short time.

When a sodium atom is in an excited state it solely stays there for round 17 billionths of a second earlier than firing out a yellow–orange photon.

However, within the aurora, lots of the oxygen atoms are created in excited states with no “allowed” methods to chill out by emitting gentle. However, nature finds a manner.

The inexperienced gentle that dominates the aurora is emitted by oxygen atoms stress-free from a state known as “¹S” to a state known as “¹D.” This can be a comparatively sluggish course of, which on common takes nearly an entire second.

In reality, this transition is so sluggish it will not normally occur on the type of air stress we see at floor degree, as a result of the excited atom can have misplaced vitality by bumping into one other atom earlier than it has an opportunity to ship out a beautiful inexperienced photon. However within the environment’s higher reaches, the place there’s decrease air stress and subsequently fewer oxygen molecules, they’ve extra time earlier than bumping into each other and subsequently have an opportunity to launch a photon.

For that reason, it took scientists a very long time to determine that the inexperienced gentle of the aurora was coming from oxygen atoms. The yellow–orange glow of sodium was identified within the 1860s, however it wasn’t till the Nineteen Twenties that Canadian scientists discovered the auroral inexperienced was as a consequence of oxygen.

What makes the pink gentle?

The green light comes from a so-called “forbidden” transition, which occurs when an electron within the oxygen atom executes an unlikely leap from one orbital sample to a different. (Forbidden transitions are a lot much less possible than allowed ones, which suggests they take longer to happen.)

Nonetheless, even after emitting that inexperienced photon, the oxygen atom finds itself in one more excited state with no allowed leisure. The one escape is by way of one other forbidden transition, from the ¹D to the ³P state—which emits pink gentle.

This transition is much more forbidden, so to talk, and the ¹D state has to outlive for about about two minutes earlier than it might probably lastly break the foundations and provides off pink gentle. As a result of it takes so lengthy, the pink gentle solely seems at high altitudes, the place the collisions with different atoms and molecules are scarce.

Additionally, as a result of there’s such a small quantity of oxygen up there, the pink gentle tends to seem solely in intense auroras—like those we have now simply had.

This is the reason the pink gentle seems above the inexperienced. Whereas they each originate in forbidden relaxations of oxygen atoms, the pink gentle is emitted rather more slowly and has a better probability of being extinguished by collisions with different atoms at decrease altitudes.

Different colours, and why cameras see them higher

Whereas inexperienced is the commonest colour to see within the aurora, and pink the second most typical, there are additionally different colours. Specifically, ionized nitrogen molecules (N₂⁺, that are lacking one electron and have a optimistic electrical cost), can emit blue and red light. This could produce a magenta hue at low altitudes.

All these colours are seen to the naked eye if the aurora is vibrant sufficient. Nonetheless, they present up with extra depth within the digital camera lens.

There are two causes for this. First, cameras take pleasure in an extended publicity, which suggests they will spend extra time gathering gentle to supply a picture than our eyes can. Consequently, they will make an image in dimmer circumstances.

The second is that the colour sensors in our eyes do not work very effectively at midnight—so we are likely to see in black and white in low gentle circumstances. Cameras haven’t got this limitation.

To not fear, although. When the aurora is vibrant sufficient, the colours are clearly seen to the bare eye.

This text is republished from The Conversation below a Inventive Commons license. Learn the original article.