"As the bomb explodes it emits a powerful wave of gamma rays. As this energy release hits the upper atmosphere it creates a electrical disturbance know as the Compton Effect. The intensity is magnified. View it as a small pebble rolling down a slope, hitting a larger one, setting that in motion, until finally you have an avalanche.
At the speed of light this disturbance races to the earth surface. It is not something you can see or hear, in the same way you don’t feel the electrical disturbance in the atmosphere during a large solar storm.
For all electrical systems though, it is deadly…
... In far less than a milli second the entire power grid of the United States, and all that it supports will be destroyed…” (Source)