An aurora is a phenomenon that has long been appreciated by human civilization. In 1619, while trying to explain this events, Galileo Galilei (1564-1642) first used the term Aurora Borealis after the Roman goddess of the morning, Aurora and the Greek god of the north wind, Boreas to refer to this mysterious lights. Although his explanation was wrong this two words reached nowadays and most people dream about going somewhere close to the poles to see it. This events are possible to see in the northern and southern hemispheres and they are called Aurora Borealis (or northern lights) and Aurora Australis (or southern lights) respectively.
Now it’s time to explain it!
We have to go as far as the Sun to build our understanding of this phenomenon. As some of you may know, this huge hot plasma sphere has strong magnetic fields which are constantly changing. This magnetic fields are responsible for a whole bunch of solar phenomena which can change dramatically what we call the space weather.
So, how can all these phenomena relate to the Auroras? Let me try to explain it! One of those solar phenomena is a constant stream of plasma with a magnetic field embedded in it that is released from the Sun’s upper atmosphere called solar wind. As you all know the weather on Earth is not always good and steady; there are storms with lightning, rain, wind… The same happens to the Sun. Of course you are not expecting to see rain in the Sun (!) but it also has his whole bunch of phenomena that could create what we call Solar Storms. From time to time, Solar Flares and Coronal Mass Ejections (CME) release unusual large quantities of plasma and magnetic field into the solar wind, as you can see on the video that follows.
Video: A series of CME that occurred in August of 2010
Depending on what area of the Sun these events occur, the solar wind shock waves may travel through space towards Earth reaching it in about 1-2 days.
WOW! Could this be dangerous? Could this be lethal? Yes, it could! It is one of the main concerns about a manned mission to Mars or other planets. Luckily we are protected by Earth’s magnetosphere, the area around the Earth where charged particles are controlled by its magnetic field, as the video below illustrates.
Video: Particles released by a CME flowing around the Earth
When these solar wind shock waves approach Earth they interact with Earth’s magnetic field compressing the magnetosphere and increasing the amount of plasma moving through it. While most of the charged particles carried by the solar wind are deflected by the Earth’s magnetic field, some are able to penetrate the atmosphere through the polar cusps – creating a daytime Aurora – and others are able to penetrate the magnetosphere on the night side and then directed back to Earth along the magnetic field lines and precipitate into the upper atmosphere in the polar regions, as you can see illustrated by the picture below.
When this high energy charged particles reach the upper atmosphere they collide with oxygen and nitrogen atoms and molecules. These collisions transfer energy to oxygen and nitrogen raising them to an excited state.
The energy does not stay there forever and it is precisely when it is released and the atom return to their ground state that light is emitted as a photon (check the schematic below) and provide us the opportunity to see this amazing effect.
The lights are shown as diffuse glow or as curtains of parallel rays aligned with the local direction of the magnetic field. The colours you can see may vary depending on the type of collision and how high they occur. Typically, interactions with oxygen produce green or red colours and interactions with nitrogen blue or red.
I challenge you to put this beautiful show on your bucket list and share your experience and pictures in the comments bellow.
BONUS: Everything you need to know to book a trip to see the Aurora Borealis phenomenon here. And, also, a dramatic time-lapse video of spectacular auroras for your entertainment.