期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：4
页码：990-994
DOI：10.1073/pnas.1417741112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceWhat sets the strength of a planet's magnetic field? Theory suggests that there exists a "sweet spot" for magnetic field generation, in which the constraining influences of the Coriolis force (from planetary rotation) and Lorentz force (from the magnetic field) partially cancel, and the convective flow that generates magnetic energy is maximally efficient. However, this predicted optimal state, termed the magnetostrophic regime, has not yet been observed in computational dynamo simulations and has never been tested in a real, turbulent liquid metal, and so its existence has recently been called into question. Here, we report the first-ever, to our knowledge, turbulent magnetostrophic convection experiments. We observe that the magnetostrophic regime is, in fact, maximally efficient, substantiating the application of magnetostrophic theory to planets. The magnetic fields of Earth and other planets are generated by turbulent convection in the vast oceans of liquid metal within them. Although direct observation is not possible, this liquid metal circulation is thought to be dominated by the controlling influences of planetary rotation and magnetic fields through the Coriolis and Lorentz forces. Theory famously predicts that planetary dynamo systems naturally settle into the so-called magnetostrophic state, where the Coriolis and Lorentz forces partially cancel, and convection is optimally efficient. Although this magnetostrophic theory correctly predicts the strength of Earth's magnetic field, no laboratory experiments have reached the magnetostrophic regime in turbulent liquid metal convection. Furthermore, computational dynamo simulations have as yet failed to produce a magnetostrophic dynamo, which has led some to question the existence of the magnetostrophic state. Here, we present results from the first, to our knowledge, turbulent, magnetostrophic convection experiments using the liquid metal gallium. We find that turbulent convection in the magnetostrophic regime is, in fact, maximally efficient. The experimental results clarify these previously disparate results, suggesting that the dynamically optimal magnetostrophic state is the natural expression of turbulent planetary dynamo systems.
