Sometimes protons and electrons, which are particles smaller than an atom, manage to get enough energy to penetrate the ionosphere, the atmospheric layer characterised by its ionised state of matter. When they do so, they collide with the neutrals in our atmosphere and voilà! Beautiful auroras dancing above our heads.
Studying the shape and structure of the auroral oval is fascinating and remains very challenging, even though auroral displays have been observed since ancient times. While for the Sami populations auroras were felt as the presence of dead souls, for the Vikings they were felt as the presence of God.
The driver of the aurora
For many years humankind has wondered about this phenomenon and tried to explain it. With the advent of space technology, we are now able to observe the aurora from space and we have come to understand more and more about it.
To get enough energy to precipitate in the dayside and nightside ionosphere, particles need to be accelerated. Our star, the Sun, energy source of every type of life, is the driver of this process.
Earth, Jupiter, Saturn, Uranus and Neptune are known for having strong aurora, as well as one of Jupiter’s moon, Ganymede.
The Sun is and gives us energy, and the solar wind, a continuous stream of charged particles constituted mainly by protons and electrons, is constantly impinging on the Earth’s magnetosphere. The Earth’s magnetosphere is shaped by the Earth’s magnetic field, which is acting as a shield, protecting us from the constant flux of particles which could otherwise strip our atmosphere apart.
The effects of the solar wind on the Earth’s magnetosphere
The solar wind carries along the Sun’s magnetic field, which after leaving the Sun, is called interplanetary magnetic field. When the solar wind’s magnetic field is oriented in the opposite direction with respect to the Earth’s magnetic field, a process called magnetic reconnection occurs.
Magnetic reconnection converts magnetic energy to kinetic energy, and accelerates the surrounding particles. Some of these particles precipitate in the ionosphere on the dayside and after some time on the nightside, causing the aurora to take place in a band around the magnetic poles, the so-called auroral oval.
Dayside auroras are usually fainter than nightside auroras. The particles in the solar wind do not have the energy to produce the bright auroras seen on the nightside. The energy of the particles on the nightside, particles that constitute the plasma sheet, reservoir of particles ready to be ejected earthward whenever reconnection occurs, is in fact much higher.
The colours of the aurora
The characteristic visible colour of dayside auroras is red whereas nightside auroras are usually green. The reason for this difference in colour is the different energies with which particles penetrate the atmosphere: particles that are more energetic penetrate more deeply.
Since at different altitudes there are different atmospheric constituents, the interaction between the charged particles and the atmospheric neutrals will lead to different auroral colours.
When the energies of the incoming electrons on the nightside are not so high, we can have red auroras on the nightside as well. Auroras are not only red and green; their colour depends on the atmospheric composition.
Where to find auroras
The main ingredients for producing aurora are an atmosphere and a magnetosphere. Auroras do not only occur on Earth. Every planet and every planet’s moon with a magnetic field and an atmosphere can potentially produce aurora. Besides Earth, Jupiter, Saturn, Uranus and Neptune are known for having strong aurora, as well as one of Jupiter’s moon, Ganymede.
A good place to see the aurora in Norway is Tromsø, and, during quiet geomagnetic times, Svalbard offers a good spot for seeing the aurora, in particular dayside aurora.
Even though particles are always precipitating in the Earth’s high-latitude ionosphere, it is possible to see the aurora in the visible light only when it is dark and only when there are no clouds. Now that the dark season is around the corner, it is the perfect time to take a chance and take a trip up north to get our heart warmed by the auroras.
The aurora is fascinating to study and the best way to get a deeper understanding of it is to look at the skies. Satellite instruments and satellite images are excellent tools for learning about the processes that lie behind this mind-blowing phenomenon.
We are still far from understanding the many rapidly changing features seen in dynamic auroral displays, however the aurora can be explained with simple concepts unravelling the fundamental physical processes.
The true beauty is comprehensible, simple, within our heart, and can be accessed by everyone when stripped of frills. After all, in life you just have to be like a Sun’s particle, connect to the source and gain that little bit of energy needed to make you the greatest actor!
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