【英语时差8,16】为什么磁极会交换方向？（上）（在线收听）

On a previous show we discussed how, every 200,000 years or so, the north and south poles switch places. Not that the places themselves move–rather, the entire magnetic field of the earth flips around, resulting in the pole we now call “north” being on the southern tip of the planet, and vice versa. Why does this happen? Although it seems pretty solid, the outer core of our planet is in a molten state. That means it’s partway between what most folks would call liquid and solid. Much of this molten interior is iron and nickel. These are elements that conduct electricity very well. It’s the fluid metals that move about inside earth that generate a magnetic field around our entire planet. Although from close up these metals are bubbling like hot soup, you could also say that in general their motions follow a pretty steady pattern. That’s why the magnetic field on earth is pretty stable.