How do you determine the Ionosphere density?
"Density" is a ratio of mass to volume.
You sample a given volume in the ionosphere and measure its mass. The amount of mass divided by the volume defines its density. The sample is most accurately taken as a complete physical volume, but it may also be inferred from analysis of elements in the ionosphere, assuming that you really know all of the primary ones.
For example, if I accept the level of error by ignoring elements or gases that are known to be very low (trace amounts) at sea level, I might simply analyze a particular volume for oxygen, nitrogen, argon and carbon dioxide directly in the ionosphere. Adding their masses would give me the volume mass with reasonable accuracy.