Low-altitude EMP Can a low-altitude EMP exist from a detonated nuclear weapon or is the higher altitude required for propagation of the relevant fields necessary to induce destructive currents?
 A: A high altitude is necessary to get an effective range. An EMP activated at low altitude will only effect electronics close to the source, and if that source is a nuclear bomb there won't really be many electronics left to effect anyway. The point of an EMP is to destroy or damage electronics while leaving the target physically intact; the high altitude of the detonation grants greater range to the former purpose while allowing the shockwave to dissipate in the atmosphere, acheiving the latter.
A: As Asher said, low-level EMP does happen and the range is reduced.
The EMP effect is mostly limited to things within the horizon, the earth blocks the effect beyond that. (Maybe there can be some diffraction into things that are hidden by the horizon or by hills, but that shouldn't be a great big effect.)
Also, when the EMP effect is downward, it goes through a minimal amount of air. Going mostly sideways it can go through a whole lot of air. That can reduce it some.
So a big, very high nuke can have a strong EMP effect across a whole hemisphere. 
Ignoring the air, the angle covered by the USA is about pi/8. So the height of a nuke that would affect the whole thing would be $4000(\sec(\pi/8)-1)$, about 330 miles. Still pretty high.
To cover half the USA it should be about 80 miles. 
