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.