File:High altitude EMP.gif

English: Background information on high altitude EMP from testing is from http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html Note that the actual peak EMP at ground zero depends also on the intensity and hence the rise time of the prompt gamma rays, and the natural magnetic field strength at the altitude of interest, so these are just approximate curves. The blue pre-ionisation curve shows how gamma rays from the primary stage of a nuclear weapon can reduce the later stage EMP by partly shorting it out, via pre-ionising the high altitude atmosphere (which allows a conduction current to immediately oppose the EMP Compton current of electrons). The full facts are secret, but these unclassified curves give a good idea of how bomb yield and burst altitude influence the maximum peak EMP on the earth's surface.