Since the beginning of times, the aurora in the northern sky causes awe, angst and amazement. How these charged particles can appear and color the sky with green and red tones and flicker like curtains? Let's consider the mechanics of this natural and physical phenomenon.

     The aurora australis is a luminous phenomenon taking place in high atmosphere. It is refered to as aurora borealis in the northern hemisphere and aurora australis in the southern hemisphere. Such a phenomenon takes mostly place under the highest lattitdes close to magnetic poles of the planet.

     The Sun generates particles which are spread into space and usually referred to as solar wind. When this flow of particles reach Earth, the magnetic shield of our planet, the magnetosphere deflects it, avoiding these particles to enter the atmosphere. In case of more violent wind, e.g. when a solar eruption occurs, some of these charged particles (protons and electrons) following the magnetic field of the Earth up (or down) to the poles excite the constituents of the upper atmosphère, the ionosphere. This electrical reaction generates ionized clouds which emit light: an aurora appear, its colour depending on the ionised atoms involved in the reaction. This explains why the phenomenon evolves in time with variation of shapes and colours. This usually occurs at an altitude of about 80 to 1000km. At lowest altitudes, the curtains will tend to have a reddish colour while they'll be more greenish or yellowish at higher altitudes.

     An aurora is not fixed in space, opposite to rainbows. This is a moving phenomenon, usually starting by an arc of light (an aroral arc) followed by beams with varying speads. We can sometimes observe light adopting the shape of a curtain blown by a soft wind. Le power of this light is variable and the phenomenon's length range from a few to many dozen minutes. On exceptionnal circumstances, auroras can be seen under Mediterranean skies, e.g. during very violent solar activity events.

     Scientists only started to study the phenomenon from the 17th century: in 1621, French astronomer Gassendi chose to name this phenomenon as aurore boréale after observing in even in the South of France. In the following century, the British astronomer Halley first suggested that magnetic fields might be involved in the origin of such events. The first reproduction in a laboratory of similar lights will only occur in 1896 by Birkeland. From 1957, the space conquest helpts to better understand these auroras on Earth, but we now know that similar event occurs on other planets such as Jupiter, Saturn or Neptune.