| The Sun seems immutable, identical to itself, but this
is only an illusion. Actually the Sun is subject to an
intense activity (picture 1 &
video 1), a perpetual boiling (video
2) continuously disturbs its surface. Perhaps
the most famous demonstration of this activity is the
sunspots (picture 2 & 3).
By calculating the international index of the sunspots,
it is possible to highlight the cyclic character of
the solar activity. And indeed, roughly every eleven
years, the Sun goes through a maximum of its activity.
This is characterized by significant modifications of
the structure of the atmosphere
and a strong increase in the frequency and intensity
of solar flares (picture 4 &
5).
This cyclic solar activity has considerable effects
on the Earth, in particular on human activities.
For example, due to the heating and the dilatation
of the higher part of the Earth's atmosphere, satellites
that travel in low orbit
around the Earth have their orbital parameters modified
and their lifetime shortened. This phenomenon is caused
by the radiation increase that accompanies the progressive
entry of the Sun in the active phase of its cycle.
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