Actually not really.
40-400 KM high depending on nature and gamma yield of the device. 40 KM is barely 26 miles high. That puts it above the earth’s magnetic field which is about 20-40KM. Nuclear EMP is part of the “Compton effect”. Compton scattering is of course the interaction between the nuclear event and the magnetic field.
Near-surface detonations are localized of course WRT EMP. Interestingly intermediate altitudes don’t induce the Compton effect (the accelerator and expander of the pulse) due to atmospheric absorption of energy.
100 kilotons @ 40 KM would do the trick, line of site, inverse square rules in effect. Larger and higher just extends the line-of-sight effects and intensity.
Multiple fission/fusion devices, lower altitudes would easily disable/disrupt infrastructure serving the population centers of the US along the coasts.
Take a look here and here:
http://www.fas.org/nuke/intro/nuke/emp.htm
http://www.thespacereview.com/article/1549/1
EMP nukes are gamma ray optimized as opposed to ‘blast’ yield optimized. The Gamma rays induce high energy electrons through interaction/excitement of/with the Earth’s magnetic field to produce the HEMP and SHREMP which peaks in nanoseconds and lasts about one millisecond. Secondary effects may last for more than a second, and these are more like a lightning strike— induceing current (E2) in very long electrical conductors (electrical and phone lines, rail lines, similar).
What’s debatable is the direct effects of nuclear EMP on modern devices.
BUT, we have a ‘thin blue line’ of sorts in our just in time world, and a three-day or three week shutdown of say just15% of critical infrastructure is likely to have DEVASTATING effects on our civility. ESPECIALLY if it’s our electrical and transportation infrastructure.
p.s. — to agree with your post - a very high altitude, multi-megaton burst, gamma ray-optimized gizmo puts us back in the 13th century for years, and the Iranians can’t yet get there.
It would have to be done at night for long range emp for the solar wind on the dayside thins out the magnetic field lines of force on the day side, squashing more of the field to the night side. Hence reduced propagation effects on the day side meaning, you’d have to explode emp devices closer to the surface on the dayside to get any emp at all.