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When I read some papers about MHD dynamo theory, there is always a $\tau$ that means convection overturning time. What does convection overturn mean?

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The convective overturn time is the typical timescale for a convective cell to rise in a gas. Imagine a "lava lamp" - it's the time for one of the blobs to rise from its lowest to highest point.

I am most familiar with its use in stars, where convective energy transport is modelled using a mixing length. This posits that the typical height travelled by an adiabatic convection cell is some multiple $\alpha$ of the pressure scale height $H_p$ (the typical length on which the internal pressure changes significantly).

The convective overturn (or turnover) time is then $\alpha H_p/v$, where $v$ is the velocity at which convective cells rise.

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  • $\begingroup$ Thanks very much, I am a freshman in this field, could you recommend a book or something else about this? $\endgroup$
    – Lack Coma
    Dec 20, 2017 at 9:06
  • $\begingroup$ @LackComa It is an output from a theoretical model - a way of characterising the convection. I am only familiar with it from research papers that can be found by searching on "convective turnover time". $\endgroup$
    – ProfRob
    Dec 20, 2017 at 10:33

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