Final week, an enormous photo voltaic flare despatched a wave of energetic particles from the Solar surging out by means of house. 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.
This spectacular spike in auroral exercise seems to have ended, however don’t be concerned should you missed out. The Solar 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 images, you is perhaps 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.
Acquired a needle in a haystack picture. It disappeared simply as quick. I’m nonetheless speechless. Aurora in Hawaii pic.twitter.com/HzKVWTHbnI
— Jacobvandervelde.eth (@JACOBJMV) Could 12, 2024
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 Solar on a regular basis, however there are extra throughout instances of higher photo voltaic exercise.
Most of our environment is protected against the inflow of charged particles by Earth’s magnetic subject. 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 higher environment, they cut up the oxygen molecules (O₂) into particular person atoms. Ultraviolet gentle 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 method that may “relax” by giving off power within the type of gentle.
What makes the inexperienced gentle?
As you see in fireworks, atoms of various parts produce totally different colors of sunshine when they’re energised.
Copper atoms give a blue gentle, barium is inexperienced, and sodium atoms produce a yellow–orange color 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. Nonetheless, nature finds a method.
The inexperienced gentle that dominates the aurora is emitted by oxygen atoms enjoyable from a state known as “¹S” to a state known as “¹D”. It is a comparatively sluggish course of, which on common takes virtually an entire second.
Actually, this transition is so sluggish it will not often occur on the sort of air strain we see at floor degree, as a result of the excited atom can have misplaced power 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 may be decrease air strain and subsequently fewer oxygen molecules, they’ve extra time earlier than bumping into each other and subsequently have an opportunity to launch a photon.
Because of this, 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 recognized within the 1860s, however it wasn’t till the Nineteen Twenties that Canadian scientists discovered the auroral inexperienced was because of oxygen.
What makes the pink gentle?
The inexperienced gentle 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.)
Nevertheless, even after emitting that inexperienced photon, the oxygen atom finds itself in one more excited state with no allowed rest. 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 could lastly break the foundations and provides off pink gentle. As a result of it takes so lengthy, the pink gentle solely seems at excessive altitudes, the place the collisions with different atoms and molecules are scarce.
Additionally, as a result of there may be such a small quantity of oxygen up there, the pink gentle tends to look solely in intense auroras – like those we now have simply had.
That is why 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 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 most typical 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 optimistic electrical cost), can emit blue and pink gentle. This could produce a magenta hue at low altitudes.
All these colors are seen to the bare eye if the aurora is shiny sufficient. Nevertheless, they present up with extra depth within the digital camera lens.
There are two causes for this. First, cameras benefit from an extended publicity, which suggests they will spend extra time amassing gentle to supply a picture than our eyes can. In consequence, they will make an image in dimmer situations.
The second is that the color sensors in our eyes do not work very nicely at the hours of darkness – so we are inclined to see in black and white in low gentle situations. Cameras haven’t got this limitation.
To not fear, although. When the aurora is shiny sufficient, the colors are clearly seen to the bare eye.
Timothy Schmidt, Professor of Chemistry, UNSW Sydney
This text is republished from The Dialog underneath a Inventive Commons license. Learn the unique article.
