<p>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.</p>
<p>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 (<em>if</em> such currents exist but I don't see why not) could also <em>de</em>crease the speed.</p>
<p>So, are these scenarios realistic?</p>

CC BY-SA 4.0
2021-06-05T14:48:28.227Z
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.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-06-05T15:00:30.680Z
@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?
Deschele Schilder
CC BY-SA 4.0
2021-06-05T23:58:03.733Z
Maybe I'm being naive, but would turning solid stop it rotating?  The rest of the Earth is solid and rotates too, and [this](http://www.columbia.edu/cu/record/archives/vol22/vol22_iss1/Core_Spin.html#:~:text=The%20inner%20core%20rotates%20in,a%20whole%2C%20the%20scientists%20found.) 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?
JeopardyTempest
CC BY-SA 4.0
2021-06-06T05:43:20.897Z
@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.
Deschele Schilder
CC BY-SA 4.0
2021-06-06T06:59:22.397Z
[Looks like](https://en.m.wikipedia.org/wiki/Iron) 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...
JeopardyTempest
CC BY-SA 4.0
2021-06-06T07:01:00.373Z
@JeopardyTempest I think you won't be able to stand on your feet...
Deschele Schilder
CC BY-SA 4.0
2021-06-06T17:00:01.307Z
@Barbierium because of the realignment of the crust?  Yeah, totally.  Because of the estimated altered rotation rate... no, doing the math, looks like even if using 13% reduction in density (thinking that's generous, because I'm basing that on comparison of room temp solid vs melting point liquid from Wiki), that's only a 143 km loss (4%) in outer core radius.  Looking at the whole Earth size change, that's just a 41 km (0.6%) shrink in entire radius, which would equate to a 1.3% increase in rotation rate (about 20 minutes quicker).
JeopardyTempest
CC BY-SA 4.0
2021-06-06T17:22:41.560Z
That treats Earth as a uniform solid body, so not accounting for density distribution into the moment of inertia. So maybe because the density is about [double nearer the core](https://en.wikipedia.org/wiki/Structure_of_Earth#/media/File:RadialDensityPREM.jpg), that'd perhaps manage to get us nearer like a 23 hour day or such... but certainly nowhere enough to counteract gravity and tip us over (well, really the potential effect we check for is ejection, not falling over, as objects on the surface (just like the air near the ground) all are with the momentum and move in unison.
JeopardyTempest
CC BY-SA 4.0
2021-06-06T17:22:51.947Z
[These SE answers](https://physics.stackexchange.com/questions/360525/gravity-and-centrifugal-force/360527#360527) suggests centrifugal force is a trivial size for Earth currently, a relatively small increase in rotation wouldn't change that.
JeopardyTempest
<geomagnetism><earth-rotation><core>
Can changing magnetic fields or a solidifying core cause the Earth to rotate faster?
Deschele Schilder
# Can changing magnetic fields or a solidifying core cause the Earth to rotate faster?

(Asked by: username_0 on 2021-06-05)

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 *de*crease the speed.

So, are these scenarios realistic?

**Tags:** geomagnetism, earth-rotation, core

> **Comments:**
> **username_1**: 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.  
> **username_0**: @username_1 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?  
> **username_2**: Maybe I'm being naive, but would turning solid stop it rotating?  The rest of the Earth is solid and rotates too, and [this](http://www.columbia.edu/cu/record/archives/vol22/vol22_iss1/Core_Spin.html#:~:text=The%20inner%20core%20rotates%20in,a%20whole%2C%20the%20scientists%20found.) 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?  
> **username_0**: @username_2 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.  
> **username_2**: [Looks like](https://en.m.wikipedia.org/wiki/Iron) 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...  
> **username_0**: @username_2 I think you won't be able to stand on your feet...  
> **username_2**: @Barbierium because of the realignment of the crust?  Yeah, totally.  Because of the estimated altered rotation rate... no, doing the math, looks like even if using 13% reduction in density (thinking that's generous, because I'm basing that on comparison of room temp solid vs melting point liquid from Wiki), that's only a 143 km loss (4%) in outer core radius.  Looking at the whole Earth size change, that's just a 41 km (0.6%) shrink in entire radius, which would equate to a 1.3% increase in rotation rate (about 20 minutes quicker).  
> **username_2**: That treats Earth as a uniform solid body, so not accounting for density distribution into the moment of inertia. So maybe because the density is about [double nearer the core](https://en.wikipedia.org/wiki/Structure_of_Earth#/media/File:RadialDensityPREM.jpg), that'd perhaps manage to get us nearer like a 23 hour day or such... but certainly nowhere enough to counteract gravity and tip us over (well, really the potential effect we check for is ejection, not falling over, as objects on the surface (just like the air near the ground) all are with the momentum and move in unison.  
> **username_2**: [These SE answers](https://physics.stackexchange.com/questions/360525/gravity-and-centrifugal-force/360527#360527) suggests centrifugal force is a trivial size for Earth currently, a relatively small increase in rotation wouldn't change that.  

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## 0 answers

