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I had a small quarrel about this on the physics site. A question was asked about what could cause a change in the rotation speed of the Earth. In addition to the answers given I proposed the possibility that if the liquid core would solidify this would result in an increasing rotation speed.

I thought the same could happen if the matter currents giving rise to Earth's magnetic field changed direction, this would cause an increase in speed too. That is, if the currents have the right direction. The reversal of the currents' momentum (if such currents exist but I don't see why not) could also decrease the speed.

So, are these scenarios realistic?

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    $\begingroup$ Under conservation of angular momentum, only a change in moment of inertia can change the rotation speed. A reversion of current direction will not lead to a change in MOI. $\endgroup$ Commented Jun 5, 2021 at 14:48
  • $\begingroup$ @AtmosphericPrisonEscape But to conserve angular momentum wouldn't a change in internal matter momenta cause external momenta to change? If the core starts rotating slower, doesn't that mean that the outer parts start rotating faster? $\endgroup$ Commented Jun 5, 2021 at 15:00
  • $\begingroup$ Maybe I'm being naive, but would turning solid stop it rotating? The rest of the Earth is solid and rotates too, and this suggests at a very similar rate? If the inner core suddenly stopped spinning, you'd think there'd have to be some vicious frictional torque there that keep any big discontinuity from forming, as gravity is going to ensure contact remains tight? $\endgroup$ Commented Jun 5, 2021 at 23:58
  • $\begingroup$ @JeopardyTempest I meant that if liquid turns solid then normally the volume is reduced. So the radius of the Earth will decrease. So the Earth's rotation will increase. $\endgroup$ Commented Jun 6, 2021 at 5:43
  • $\begingroup$ Looks like iron (and nickle) are maybe 15% less dense as liquids. In the grand scheme of that being only part of the Earth, can't envision it'd be a gargantuan change. I'm a wee bit more concerned about how the mantle and crust adjust to the new shape of this proposed slight shrinking of the core... $\endgroup$ Commented Jun 6, 2021 at 6:59

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