Many theories about ghosts are linked to the electromagnetic field. Putting aside all debate about whether or not ghosts actually are a product of electromagneticism, what sort of event or phenomena - either naturally occurring or man made - could a writer use to justify increased ghost activity because of a much stronger electromagnetic field over a vast area? At least large enough to encompass an entire country (like the US) if not the whole world? If this could be a cause that doesn’t dissipate quickly, all the better.
With the exception of gravity, everything we experience is a product of electromagnetism.
And the electric field is tied in to space-time. To change the strength of the field would mean changing the speed of light. The charge on the electron would change. Atoms might not be able to form. It’s a real mess.
Also, you probably won’t find any support here for electromagnetic ghosts, or ghosts of any sort. This is a pretty skeptical crowd, and we (good citizens of Missouri) want to see the actual evidence first.
That said, hey, ball lightning and St. Elmo’s fire. Electricity does weird stuff.
Finally, if you’re writing science fiction, go for it! Change anything you want, and you don’t have to justify it. Anything goes! Just ask H.G. Wells!
Increasing the strength of electric fields is easy. Just increase the amount of charge that’s generating it. Or get closer to the source.
If there were anything in science - any field, any discipline, any theory - that could legitimately justify ghosts, a billion kooks would be blazing it across the Internet at the top of their lungs.
They don’t, so there isn’t.
A big solar storm might do the trick. Of course, no scientists would attribute it to ghosts – but then, most people on earth aren’t scientists…
I’ve seen a ghost, couple weeks after my father passed away, he appeared to me in a vision telling me to co-operate with my mother.
But that’s just biochemistry …
It doesn’t even need to be the real reason, though. Basically just a science-y possible theory that “experts” offer for why the hell we’re suddenly overrun with ghosts.
I know making a point like this in this context is absurd but …
I don’t think merely increasing the strength of an EM field is enough. You need an increased gradient. It’s the difference between high and low fields that allows anything feeding off that source to be able to be more energetic.
It’d be like putting more water behind a hydro dam without increasing the height of the dam. The generators are still going to produce the same amount of power per minute.
An increase in the strength of the Earth’s magnetic field would be a good candidate due to the field being worldwide, and due to its generating mechanisms being both imperfectly understood & inaccessible. So, the “why did it get stronger” problem can be pretty much handwaved away with “We don’t really understand how it works very well in the first place”.
The trouble with “electromagnetic field” is that it is a little bit loosely defined here. Just what is meant by a “much more powerful field” and espcially how this is meaningful over an entire continent.
One might note that linking ghosts to scientific ideas seems to track the state of popular science in a manner a bit reminiscent of paranoid delusions. There was a time when the afflicted would claim that they were the victims of remote mind control using radio waves. Before that it was more likey to be witchcraft, and since then, computers. Ghosts are likely to be linked to whatever the popular science du-jour is.
The Earth has a magnetic field. For almost all useful purposes this field is static. It also has an electrostatic field, one that is a lot more messy, but their is a potential difference between the ground and object above the ground that increases with altitude.
As Ignotus notes above, the thing that does cause real injection of electromagnetic energy on a global scale is a solar storm. A really big coronal mass ejection can cause large scale and very powerful currents to flow in the upper atmosphere, powerful enough that they can knock out power transmission systems. The flow of charged particles in the upper atmosphere and further up is a complex and freaky thing. Recently there has been confirmation of large flux tubes running around the planet. If you were looking for a vaguely tenable plot device for a story-line about ghosts suddenly appearing, this would probably be about it. Normally they do only last for a day or so. But an unusually very active sun might suffer a number of CMEs, and by pure bad luck more than one might hit us (most miss).
Ever since the Victorians got heavily into psychic research, tons of possible science-y explanations have been introduced for every type of manifestation. The pairing of ghosts and electromagnetic fields is one of them. Why in this or any other world should there be a connection between the two?
So the addition of more nonsense wouldn’t change anything. The only change anyone would latch onto is an explanation with some real science behind it. But that is a null set.
I think you could go with increased influence of solar winds due to waning magnetic fields due to incipient flip of the earth’s magnetic field. This is a good “something going on in the core” thing. They say a magnetic field flip is coming soon (any day now 
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Or go with unusually active solar cycle. There have been periods of greater and lesser sunspot activity in the past (see Maunder minimum). Then you could use this to explain prior episodes of mass hauntings or evil kingdoms or whatever.
Alternatively, consider solar manipulation by alien intervention (might help but would probably hurt your plot, though).
For a good SF book about ghosts in the sun, see ‘Sundiver’ by David Brin.