The charge density will increase with increasing height because it is the radiation from the sun (daytime) or cosmic rays (night-time) that creates the particles that exist in the ionosphere.
Charge density of the ionosphere is a result of ultra-violet and X-ray solar radiation or cosmic rays striking air molecules in the upper atmosphere resulting in ions and free electrons in a plasma.
The solar energy striking the outermost air molecules in the thinnest air will have the greatest effect, resulting higher energy free electrons.
The free electrons can exist for some time before being re-captured by a nearby ion. This process is repeated over uncountable air molecules, so there are always a shield or plasma of electrons and ions.
Plasma is considered to be the forth state of matter. The plasma has a significant effect on the transmission of radio waves around the world.
There are images here: http://solar-center.stanford.edu/SID/activities/ionosphere.html