Thursday, October 14, 2021

The sky above the Azores shows us a phenomenon always present but difficult to see: the airglow

 It is one of the many events that can happen when the Earth is hit by particles from the Sun. Here we are talking about ultraviolet radiation (100-400 nm) that hits, exciting them, atoms and molecules between 80-100 km in altitude.  All these interactions can generate photons that emit at different wavelengths depending on the affected atoms and reactions generated: blue, green, red...

 You must have remembered the auroras and you are right!  The phenomena are always generated by the encounter between radiation / solar particles in our atmosphere.  However, there is a difference: in fact, auroras come from the collision of charged particles (electrons and protons from the solar wind) that follow the Earth's magnetic field, with the ionosphere, generating energy in the form of light at various wavelengths.  In airglow, the phenomenon is diurnal, can happen at any latitude and is of the ′′ chemical type because the short ultraviolet wave produces these glows following chemical reactions (chemoluminescence).

 This particular photo is beautiful because we don't have generalized brightness, but the airglow's arrangement is given by gravity waves, beware that we're talking about fluid dynamics not relativity!  These gravitational waves form in fluids when they are disturbed, for example, by contact with others but by varying densities, temperatures or because of an external event (a rock in a pond! ).  We know that nature tends to balance, so when a fluid is disturbed to meet a front with different characteristics, the hydrostatic pressure or the force of gravity will tend to generate oscillations around the equilibrium point, with phase and period dependent on the Characteristics of the two fluids, until restoring their balance.


 Credits: Miguel Claro

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