The study of the upper layers of the atmosphere began as a direct consequence of Marconi’s first experimental transatlantic radio transmission in December 1901, with the aim of providing a theoretical basis to Marconi’s practical result, which most people deemed impossible.

Marconi’s exploit could be in fact justified only by admitting the existence of the Ionosphere, that is an electrically conducting layer of the atmosphere which works as a mirror able to reflect electromagnetic waves.

Such layer was actually discovered at an altitude of 100 km (E layer).

Later on, other upper and lower reflecting layers were identified by using higher frequencies, in particular the F layer (or Appleton’s), at an altitude of about 250 km, which is very important to long-distance radio communications.

 

Come si propagano le onde elettromagnetiche

entry Ionosfera in the Treccani Encyclopedia