Visibility of a polar aurora :
When we talk about visibility, we think at the light that our eye receives. Actually, the light corresponds to a cosmic particle : the photon. The photon is in fact the light's messenger, and constitutes what we call the electromagnetic radiation. So our eye can be considered as a photon sensor, but a very complex one, as it can distinguish the colours. Nevertheless, we cannot, at our modest level, determine an emitted photon's quantity from any light source (a polar aurora for example) in real, that's why we will have to use a model, much more reachable for us.
But before that, we will first make an inventory of what determines the aurora's intensity and then we will tell you about what influences its vision.
A magnificient sight offered by nature...
On one hand, as it is explained in the phenomenon part (previous page), the sun has a cyclic activity.
According to its position in this cycle, there are more or less sunspots on its surface.
Now, the more sunspots there are, the more provoked solar flares occur.
The more these solar flares occur, the more solar floods and thus particles there are.
As we have more of them, the particles will be more numerous to enter the terrestrial atmosphere and to collide, thus interact with the existing gas.
As the interactions are more numerous, the produces light's quantity after the excitation will be more important too.
And finally, the more light there is, the more the so-created auroras will be important, in length as in intensity.
So, we realize that the sunspot's quantity is responsible of the auroras intensity.
| Sunspots and days with polar auroras by years |
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| Copyright : Astrosurf |
On the other hand, we have three possible types of interaction in the high atmosphere between gas and particles.
Some collisions are only an ions' transfer.
Others, called reflective, send the solar wind's particles towards the opposite hemisphere.
The third interaction type, which is the most important quantitatively, concerns neutral gas.
Solar wind's issued electrons heat the high atmosphere's electrons by friction.
The ions, atoms and molecules which are excited, get normal again by emitting a visible radiation.
As far as we are concerned, we will only take care of the last interaction.
We said before that the solar winds' importance determined the number of interactions.
We need to introduce something else : the more neutral gas (nitrogen and oxygen) there is in the atmosphere, the more the collisions, and then the produces light will be important.
So we are now able to affirm that sunspots and high atmosphere's neutral gas concentration are the factors which influence the intensity of a polar aurora, and which are responsible of its formation.
