Magnetic Field of the Earth

The Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. The problem with that picture is that the Curie temperature of iron is about 770 C . The Earth's core is hotter than that and therefore not magnetic. So how did the Earth get its magnetic field?

Magnetic fields surround electric currents, so we surmise that circulating electric currents in the Earth's molten metallic core are the origin of the magnetic field. A current loop gives a field similar to that of the earth. The magnetic field magnitude measured at the surface of the Earth is about half a Gauss and dips toward the Earth in the northern hemisphere. The magnitude varies over the surface of the Earth in the range 0.3 to 0.6 Gauss. 磁场环绕着电流，因此我们推测，地球熔融金属核心中的环形电流是磁场的来源。一个电流环所产生的磁场类似于地球的磁场。地球表面的磁场强度约为半高斯，并在北半球向下倾斜。磁场强度在地球表面的范围是0.3到0.6高斯。

The Earth's magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles. The fact that aurora have also been observed on Jupiter and Saturn is evidence that they also have sizable planetary magnetic fields.

The north pole of a compass needle is a magnetic north pole. It is attracted to the geographic North Pole, which is a magnetic south pole (opposite magnetic poles attract). Although the bar magnet in the illustration gives a useful picture of the nature of magnetic field lines, the Earth's magnetic field arises from a collection of effective electric current loops, and not from the magnetic properties of the iron of the core material.

The Dynamo Effect

The simple question "how does the Earth get its magnetic field?" does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term "dynamo effect" or "geodynamo", evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. As this hot fluid moves away from the hotter solid inner core, it is moving in a rotating system and is subject to the coriolis force. The coriolis force generates circulation of the fluid relative to the Earth. This means the electrically conducting material (iron) moves relative to the earth's magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. A collection of electric current loops generated by convection circulation in the molten outer core along with the effects of the rotation of the Earth is presumed to be the source of the sustained magnetic field. Large-scale computer models are approaching a realistic simulation of such a geodynamo. An overview of the influences that make the simulation so difficult may be found in the wiki Dynamo Theory.

Magnetic Field Reversals

Bjornerud tells the story of U.S. Navy vessels towing sensitive magnetometers across the ocean floor in the 1950s as a part of a "cold war" strategy of finding ways of detecting Soviet submarines and concealing theirs. 波乔内鲁德讲述了20世纪50年代美国海军舰艇在海底拖拽敏感磁力计的故事，这是冷战时期一种寻找探测苏联潜艇的方法，同时也在隐藏自己的潜艇。 When the normal magnetic field of the Earth was subtracted, there were variations on the order of 10% from the magnetism of the sea floor rocks which showed reversals of direction. 当减去地球的正常磁场后，海底岩石的磁性显示出方向反转，其变化幅度约为10%。

The graphic above from the USGS shows the pattern of reversals out to about 4Myr as determined by potassium-argon dating. This set of measurements in opposite directions from the East Pacific Rise shows remarkable symmetry and contributes to our picture of plate tectonics.