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answers-03-24-2011 - Answers to Physics 176 One-Minute...

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Answers to Physics 176 One-Minute Questionnaires Lecture date: March 21 and March 17, 2011 Where does Earth’s magnetic field come from? It is known to arise from something called the “dynamo theory”, you can read about it in the Wikipedia article with that title. The physics involves a combination of fluid dynamics, the Maxwell equations, and nonlinear dy- namics and the technical details lie beyond what most undergraduate physics majors are prepared to understand. Briefly: the magnetic field is formed and sustained by a combination of convection of an electrically conducting fluid (for the Earth, liquid iron) and of rotation arising from the Earth spinning on its axis once every 24 hours. But the exact mechanism of how the field arises remains only roughly understood. For example, it is difficult to construct experiments involving rotating convecting liquid metal that operate in the same regime as the Earth’s iron core, and the mathematics is nonlinear and difficult so that supercomputer simulations are needed, and it turns out that the parameters corresponding to the Earth’s core lie barely within reach of the most powerful computers available. It also turns out that one needs some small initial field magnetic field to get things going, and the origin of the initial field is poorly understood. It is a very good thing the Earth has such a strong magnetic field since it deflects the solar wind (consisting mainly of energetic protons, which follow the Earth’s magnetic field lines to the North and South poles) from striking the Earth’s surface, where it would harm life and scour the atmosphere away. Mars is believed to have had a liquid convecting iron core at some point, just like the Earth, but when this core solidified a billion years or so ago (Mars is one half the diameter of Earth so has a relaxation time that is 1/4 Earth’s relaxation time), Mars lost its magnetic field and the solar wind caused Mars to lose most of its atmosphere to space.
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