Final week, an enormous photo voltaic flare despatched a wave of energetic particles from the Solar surging out by means of area. 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 generally 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.
Bought a needle in a haystack picture. It disappeared simply as quick. I’m nonetheless speechless. Aurora in Hawaii pic.twitter.com/HzKVWTHbnIMight 12, 2024
This spectacular spike in auroral exercise seems to have ended, however don’t fear when you missed out. The Solar is approaching the height of its 11-year sunspot cycle, and durations of intense aurora are more likely to return over the subsequent yr or so.
When you noticed the aurora, or any of the images, you is likely to be questioning what precisely was occurring. What makes the glow, and the totally different colors? The reply is all about atoms, how they get excited – and the way they chill out.
When electrons meet the ambiance
The auroras are brought on by charged subatomic particles (largely electrons) smashing into Earth’s ambiance. These are emitted from the Solar on a regular basis, however there are extra throughout instances of larger photo voltaic exercise.
Most of our ambiance 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 ambiance 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 higher ambiance, they cut up the oxygen molecules (O₂) into particular person atoms. Ultraviolet mild from the Solar 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 mild.
What makes the inexperienced mild?
As you see in fireworks, atoms of various components produce totally different colors of sunshine when they’re energised.
Copper atoms give a blue mild, barium is inexperienced, and sodium atoms produce a yellow–orange color that you could be even have seen in older avenue lamps. These emissions are “allowed” by the foundations of quantum mechanics, which implies 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 mild. However, nature finds a manner.
(Picture credit score: AAP Picture/Ethan James)
The inexperienced mild 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 virtually an entire second.
In actual fact, this transition is so sluggish it gained’t often occur on the form 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 ambiance’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 mild of the aurora was coming from oxygen atoms. The yellow–orange glow of sodium was identified within the 1860s, but it surely wasn’t till the Twenties that Canadian scientists discovered the auroral inexperienced was as a consequence of oxygen.
What makes the pink mild?
The inexperienced mild 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 implies they take longer to happen.)
Nevertheless, 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 mild.
This transition is much more forbidden, so to talk, and the ¹D state has to outlive for about about two minutes earlier than it could actually lastly break the foundations and provides off pink mild. As a result of it takes so lengthy, the pink mild solely seems at excessive 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 mild tends to seem solely in intense auroras — like those we’ve simply had.
For this reason the pink mild seems above the inexperienced. Whereas they each originate in forbidden relaxations of oxygen atoms, the pink mild is emitted far more slowly and has the next probability of being extinguished by collisions with different atoms at decrease altitudes.
Different colors, and why cameras see them higher
Whereas inexperienced is the commonest color to see within the aurora, and pink the second most typical, there are additionally different colors. Particularly, ionised nitrogen molecules (N₂⁺, that are lacking one electron and have a constructive electrical cost), can emit blue and pink mild. This could produce a magenta hue at low altitudes.
All these colors are seen to the bare eye if the aurora is brilliant sufficient. Nevertheless, they present up with extra depth within the digital camera lens.
There are two causes for this. First, cameras get pleasure from an extended publicity, which implies they will spend extra time accumulating mild to supply a picture than our eyes can. Consequently, they will make an image in dimmer situations.
The second is that the color sensors in our eyes don’t work very nicely at the hours of darkness — so we are likely to see in black and white in low mild situations. Cameras don’t have this limitation.
To not fear, although. When the aurora is brilliant sufficient, the colors are clearly seen to the bare eye.
This edited article is republished from The Dialog beneath a Inventive Commons license. Learn the authentic article.
