Now, this will be my last off-topic message about EMP's, but here goes:
-The effects of EMP in electronics are caused by, well, electromagnetic pulse. Which is, essentially, an intense burst of electromagnetic radiation. Rapidly changing electromagnetic field induces electric current onto closed circuits, which causes erratic behaviour or, if the current is strong enough, damages components permanently.
What then causes the pulse itself? I've always thought it was the radiation from the explosion itself. If I understood youre last message right, nukephile, it would say that EMP is caused by radiation ionizing the particles in atmosphere asymmetrically, which causes differences in amounts of free charged particles in atmosphere and resulting in strong electric fields.
I think this is somehow related to what I wrote, except that I wrote my message a tad bit unclear so let me make it a bit more clear.
Let's first consider a nuke going off in vacuum, right? In nuclear reaction fissionable material releases some fotons, usually of gamma/röntgen wave lengths. Some part of the energy also goes onto kinetic energy of reaction result particles; this can also be seen as thermal energy. The gamma/röntgen burst itself counts as an EMP, though its main effects are not on electric circuits...
If there's an atmosphere around the bomb, intense radiation heats the matter around the bomb so much that it effectively ionizes quite a bit. When these particles in close proximity to bomb go through multiple absorption-emission-cycles, they change much of gamma/röntgen radiation onto more reasonable wavelengths, ie. infra-red and visible light. Also some amount of radiowaves is emitted at little further from the center of the explosion.
Also, when you detonate a nuke in upper atmosphere in the part which is called ionosphere you have to remember that the gas there is already effectively ionized. So, the punch of gamma/röntgen burst heats the ionosphere around the bomb, which causes the ions as charged particles emit electromagnetic radiation. To my knowledge, this is the phenomoneon that makes the EMP especially strong hen a nuke detonates in ionosphere.
Compton scattering on the other hand is yet another phenomenon, that relates to Röntgen diffraction and stuff... I don't think it's necessary to pull that to this matter, it just complicates things. Basically, the whole stuff is as simple as this: accelerating charged particles emit electromagnetic radiation. The bomb's intense radiation heats a wide area of ionized gas to tremendous temperatures, resulting in very fast acceleration of the charged particles in bomb's proximity. Which then shows itself to us as
a. mechanical energy in form of fast increase of the volume of the gas (shockwave)
b. Thermal energy radiating as elecromagnetic radiation of multiple wavelengths (infra-red, visible light, radio waves, microwaves and stuff).
We can continue the debate in another thread if necessary, but physically accurate portrait of EMP generation is not necessary to discuss of means to possibly destroy the Gargant, I daresay.