23.2 Geomagnetic storms and EMP: what is known and what is uncertain

These two topics attract a lot of misinformation. This is what official sources and test studies say.

Geomagnetic storm

• NOAA rates them from G1 (minor) to G5 (extreme). In a G5, according to the official scale, there can be widespread voltage-control problems, some grids may collapse and some transformers may be damaged; shortwave (HF) radio can be unusable for 1–2 days and GPS degraded for days.
• G5 storms occur about 4 times per 11-year solar cycle. Most do not cause major blackouts: operators receive alerts and take action.
• They affect the grid, not your devices. The storm induces currents in very long transmission lines. Your phone, your car, and your unplugged power station are not damaged by the storm itself.
• What you do: if NOAA issues a G4–G5 alert, charge everything, fill the tank, and review your blackout plan. You do not need more than that.

EMP from a high-altitude nuclear detonation

• A detonation tens or hundreds of miles up could produce a pulse that damages electronics and the grid over a very large area. It is a wartime, low-probability scenario, and no one can say with certainty what the real damage to modern equipment would be.

• Cars: the Congressional EMP Commission tested 37 cars (1986–2002 models) with fields up to about 50 kV/m. None was destroyed; 3 stalled while running (starting at about 30 kV/m) and were able to restart; 1 needed dashboard repair; 25 had minor faults (flickering dashboard lights) and 8 had no effect at all. The commission estimated that in a real event perhaps 10% or more of cars would suffer serious effects such as stalling. Today's cars have much more electronics and were not tested in that study: that is uncertain.
• Small electronics: unplugged devices without long cables tend to be less vulnerable than those connected to the grid. Storing a spare radio and a charger in a closed metal box (a homemade "Faraday cage") is cheap, but its real effectiveness depends on the construction and is not guaranteed.
• An EMP would be part of a nuclear attack: follow chapter 15 first. For the purposes of this chapter, the practical response is the same as in any long-duration grid failure.