There are a number of science fiction novels, stories, and TV series in the last couple decades that use EMP (electromagnetic pulse) as the basis for destroying civilization and bringing on the apocalypse.
Granted, science fiction allows for suspension of disbelief for one scientific principle. Well, at least one. Sometimes, they get away with more than one. But, getting something correct–or at least close to what is known about something–is important.
EMP is not completely understood, even today. For exhaustive details, check this link: The Space Review. After many years of testing and analysis, we have three levels of pulse from an EMP event, E1, E2, and E3. Not all testing events to evaluate EMP ended in results that are meaningful or complete.
However, some have shown some disturbing results. For example, the Test 184 by the Soviet Union in the early 1960s, with a 300 kiloton device detonated at 290 kilometers (180 miles) knocked out a 600 mile radius power grid in Kazakhstan. It was the E3 portion of the pulse that connected to the long, underground, transmission line causing several fires. There is little evidence of damage from the E1 and E2 portions of the pulse, but there were no integrated circuits (ICs) then, and the electronics and systems at the time would be unaffected by those.
Now, 290 kilometers is actually quite a ways outside our atmosphere. Putting a device that far into space to cause and EMP over somewhere is somewhat counter productive, since many of the effects of the blast and EMP would be scattered and deflected. Aside from the ethics of the Soviet Union using Kazakhstan and its people in a test of a nuclear device, there was a reason behind it. Kazakhstan has latitude and magnet field orientation similar to the continental US.
In any case, it would take about four or five similar devices exploding at that altitude–since they only affect a surface area of about 1200 miles in diameter–to completely cover the continental US and disable power distribution and generation. Whether the E1 and E2 portions of the pulse would have any impact on ICs at that distance depends on atmospheric conditions, the magnetic field, and other interference around those ICs.
The problem with this is that there are few, if any, 300 kt devices left. Today, most are megaton level nuclear warheads on systems designed to deliver the devices to specific land targets. The resulting EMP from land detonation would be limited to the area of the target. Air bursts of a few miles above a specific target would increase the area affected by the EMP but it is still limited. Granted, the power grid can carry the E3 beyond the impact zone and damage generation facilities.
As mentioned in the linked article, a crude, small, single kiloton device detonated at an altitude of about 40 kilometers (24 miles) can have a more effective EMP impact. Granted, a much smaller “horizon,” but more effective.
The article goes into much more detail, and the excerpts from the EMP Commission reports are illuminating. Also, the footnotes lead to more detail as well as history.
So, with this information, can you build a chilling, thrilling, and accurate science fiction story, novel, or script?