The northern lights seen from Kohler-Andrae State Park in Wisconsin. Credit score: Elizabeth Gamillo.
Final week, an enormous solar flare despatched a wave of energetic particles from the Solar surging out via space. Over the weekend, the wave reached Earth, and folks all over the world 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 fear in case you missed out. The Sun is approaching the height of its 11-year sunspot cycle, and durations of intense aurora are more likely to return over the following yr or so.
If you happen to noticed the aurora, or any of the photographs, you is likely to be questioning what precisely was occurring. What makes the glow, and the completely different colors? The reply is all about atoms, how they get excited – and the way they calm down.
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 occasions of higher solar exercise.
Most of our ambiance is protected against the inflow of charged particles by Earth’s magnetic area. However close to the poles, they’ll 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 approach that may “calm down” by giving off power within the type of mild.
What makes the inexperienced mild?
As you see in fireworks, atoms of various components produce completely 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 principles 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 calm down by emitting mild. However, nature finds a approach.
The inexperienced mild that dominates the aurora is emitted by oxygen atoms enjoyable from a state referred to as “¹S” to a state referred to as “¹D”. It is a comparatively sluggish course of, which on common takes virtually an entire second.
Actually, this transition is so sluggish it gained’t normally occur on the sort of air stress we see at floor stage, as a result of the excited atom could 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 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.
Because of this, 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 recognized within the 1860s, but it surely wasn’t till the Nineteen Twenties that Canadian scientists found out the auroral inexperienced was resulting from oxygen.
What makes the pink mild within the aurora?
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 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 rest. 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 may well lastly break the principles 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 look 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 colours, 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 optimistic electrical cost), can emit blue and pink mild. This will produce a magenta hue at low altitudes.
All these colours are seen to the bare eye if the aurora is shiny sufficient. Nonetheless, they present up with extra depth within the digicam lens.
There are two causes for this. First, cameras benefit from a protracted publicity, which suggests they’ll spend extra time accumulating mild to supply a picture than our eyes can. In consequence, they’ll make an image in dimmer situations.
The second is that the color sensors in our eyes don’t work very effectively 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 shiny sufficient, the colours are clearly seen to the bare eye.
This text first appeared on The Conversation. It’s republished right here below a Artistic Commons license.
