EMP pulse

EMP stands for Electromagnetic Pulse is is the result of a nuclear 
explosion.  If the bomb is not focused to specific forms of energy or 
radiation, i.e neutron bombs, then energy is released all across the 
spectrum from microwaves, light, heat, x-rays, gamma rays etc.	An EMP 
does damage by overloading the system with a huge pulse of energy across

the whole bandwidth.  One example of intense microwave radiation would
be 
putting a metal object in your kitchen microwave. Multiply the light
show 
from your 200 watt microwave to the megawatts generated by EMP and you 
can see how electronics get fried.  The only reason a nuke would be set 
off in the atmosphere and not higher up in orbit is that a lot of the 
powerful radiation (i.e. short wavelengths) tends to disperse quickly 
unless focused and since it's difficult to point a nuclear blast, to get

the most out of a bomb it needs to be relatively close to the target,
about 
200 miles up to get good coverage of 80% of the lower 48 and some of
Alaska.

--Binhan