The beaches of southern Sicily offer extraordinarily dark skies, ideal for astronomical observation. Under conditions of particular atmospheric transparency and in the absence of light pollution, a long-exposure photograph can reveal a fascinating natural phenomenon: airglow.
But what exactly is airglow? It is a faint, diffuse light coming from the Earth's atmosphere, generated by chemical reactions at high altitude, mainly in the mesosphere and thermosphere.
To simplify, airglow is due to:
the recombination of atoms and molecules (particularly oxygen and nitrogen) previously excited by solar radiation during the day;
chemiluminescence, the emission of light from chemical reactions between molecules in the upper atmosphere.
It is crucial to distinguish airglow from polar auroras: while the latter are caused by the interaction between the solar wind and the Earth's magnetic field, airglow is a constant process independent of solar activity, visible even in the absence of geomagnetic events.
Canon 6d, Sigma 14mm, f/2, iso 2500, 20 sec