The question Could we still live if Earth's spin speed slowed down by 90% assumed Earth's spin speed slowed down by undefined human activities (as if climate change was not bad enough). One answer suggested that Earth spinning slower will have weaker magnetic field, which is Not GoodTM
My question is: If that happened, what would be a plausible way to get to Earth spin faster, to get back the protection provided by Earth's magnetic field? Back to normal 24 hour days? Without destroying Earth in the process, so please no asteroid bombardment if we can avoid it.
We should increase spin slowly: even if it takes a century or two, it is OK. Because of huge energy necessary, any quick solution might melt Earth crust in the process.
And even though a slower spinning Earth was not so deadly, long nights are certainly annoying. And adding leap second every month (as we will have to in a few centuries' time) gets old soon too. We want to get Earth spinning because we want to, not because we have to. How?
XKCD cannot find an answer - maybe it is really tough.
I am not concerned about the bad effects this slow spin would have on Earth (you can add your answer to the question which inspired mine, follow the link). And "undefined human activities" cannot be reversed for whatever reason, we need to invent something new and interesting.