As described in the previous section the principal cause of the high density of free electrons and positively charged ions and molecules is photo-ionization from UV radiation arriving from space and in particular from the Sun, ignoring here other causes like energetic particles, again originating from the Sun, or radioactive elements from natural sources or fall-out pollution. Bearing in mind that the density of neutral atoms decreases with altitude and that ionization becomes weaker because of absorption, the two opposite phenomena should logically result in a
layer containing a maximum concentration of free electrons.