The upper atmosphere of the Earth responds very quickly to the Sun. Here is a brand-new app that lets you know what the temperature and density of the upper atmosphere is. It is the MSIS model on your phone!
You can look up values or draw plots of the density and temperature. Here is an example of the temperature.
It may be a pretty good vacuum up there, but it brings down satellites and produces the ionosphere that interferes with radio waves. Available free from the iTunes store. An explanation is available at the MSISatmos webpage.