A powerful enough solar flare could simulate a nuclear-initiated EMP very nicely, to the point where it could even burn out stuff on the surface. Satellites get hiccups a couple times a year because there is a flare when they are in the wrong place at the wrong time.

Imagine aura borealis at the equator at noon. If that happens, odds are the light is turning red and the big, scary, all consuming ball of swelling plasma will be reaching out to us pretty soon