The polar aurora (Part 1)

Polar aurorae have always been popular among travelers. As soon as we arrive in a country close to the polar circles, our wish is to see one. In New Zealand, it is possible to see during the autumn and winter periods. I will explain what an aurora borealis (for the northern hemisphere) and an aurora austral (for the southern hemisphere). I would explain in a second article where, how and when can we see them?

All the photos of the auroras of this article come from a photographer friend, Maxime Sacré. See his website for beautiful photos: http://www.maximesacre.com/

aurore — Aurora austral at lake Wakatipu en New-Zealand. Photo: Maxime Sacré

Formation of an aurora?

A polar aurora is “the perturbation of the magnetosphere surrounding the Earth by the solar winds” (Science Avenir). Here, it’s not clearer I think …. But don’t worry! We will define the two terms of the definition: magnetosphere and solar wind. And for this we will start with the magnetic field.

1) The magnetic field

To explain what is the magnetosphere, we must explain of what it composes, the magnetic field. If you know a little Science Fiction, you have surely already heard this term. Indeed, the magnetic field is a shield surrounding the Earth. It protects the Earth from spatial “particles” from the Sun. If this shield did not exist, the particles would exterminate all life on our blue planet.

champ magnétique — Magnetic field surronding the Earth. We can see than the shield go to the polar of Earth.

If the magnetic field is above our head, it is under our feet that we must find its origin.

The core of the Earth is composed of two parts: the core and the outer core. The core is solid and rich in iron. The outer core is liquid, consisting mainly of iron and nickel.

noyau Terre — Convection movements within the Earth. We can the convection in the outer core. (by D. Sasselov & D. Valencia, in Science, oct. 2010)

The core rotates in the liquid of outer core. This rotation causes a convective movement in the outer core. This convection will cause the swirling of the iron in the liquid of the outer core. Thus, this mixing will allow the creation of an electric current which becomes the magnetic field.

Small information on the magnetic field

At the scientific level there are two types of poles. We have the poles (North and South) which are the poles where the rotation of the Earth passes. The second type is the magnetic pole. The magnetic pole is not at the same place as the geographic pole, but at an interval of 11.5 °, ie between 500 and 600 km. The magnetic pole is important because it is such as thanks to it that one can show the North.

Indeed the North Pole is a pole of “Southern” magnetism. Which means that it will attract the poles of magnetism “north” as the tip of a compass needle. One can see this as a magnet, the “-” attracting the “+”. If the “-” is the “south” magnetism, the compass needle being the “+” will be attracted and will give the direction of the North.

A NASA study suggests that the magnetic north pole is changing course due to climate change. Indeed, the melting of the polar ice changes the mass of the Earth and this would have an impact on the direction of the North magnetic pole. It is also seen that it moves faster and faster.

2) The magnetosphere

The magnetosphere is the space surrounding the Earth between 800 and 1000km away from the surface. It is a shield composed of the terrestrial magnetic field whose goal is to protect the planet from the solar “particles” arriving on it. These solar particles, called more commonly solar winds are harmful to life. The aim of the magnetosphere is thus to protect life by deflecting the solar winds arriving on Earth. This deforms the magnetosphere by flattening the part facing the Sun and giving it a comet shape for the part opposite the Sun.

magnétosphère — Deformation of the magnetosphere by solar winds. (by NASA)

3) Solar winds

vent solaire — Solar wind on the surface of the Sun

The heat on the surface of the Sun is several millions of degrees. With such a high temperature, the collision between the particles is so violent that it forms a plasma flux of ions and electrons. As you can guess if you are a bit experience in the Science Fiction, a plasma shot is not good. Well, here it is the same thing it hurts and it can destroy life. Fortunately the magnetosphere protects us from most winds. But some still arrive to penetrate!

The polar aurora

Now that we have translated all the workings of the aurora, let us go back to the definition and explain it. The auroras are: “The disturbance of the magnetosphere surrounding the Earth by the solar winds”

One understands then that during the passage of the solar wind towards the Earth, the shield (the magnetosphere) undergoes a perturbation.

Disturbance of the magnetosphere by solar winds. (by NASA: ccmc.gsfc.nasa.gov)

Indeed, some of the particles of the solar winds are captured by the magnetosphere. They bypass the magnetic field and return through the earth’s poles. On the way, they accumulate energy. When the particles enter the atmosphere, they will excite the atoms of oxygen and nitrogen. These excited atoms will have to free themselves of this energy. Upon release of this energy, the atoms will emit photons. It is these photons that are the essence of auroras! Photons are particles that make up the light and they are the ones who will produce the light of the aurora.

Here is a video that will explain the path of solar winds in the magnetic field.

The colors of the auroras

The colors depend on the concentration of molecules of nitrogen and oxygen. Oxygen mainly emits green and red, while nitrogen emits blue, red and violet. The atmosphere has oxygen and nitrogen densities that vary with altitude, oxygen becoming denser than nitrogen above 200 km altitude, which partly explains the predominance of green in the aurora polar.