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.|
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 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. This means the electrically conducting material 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. Large-scale computer models are approaching a realistic simulation of such a geodynamo.
When energetic charged particles enter the earth’s atmosphere from the solar wind, they tend to be channeled toward the poles by the magnetic force which causes them to spiral around the magnetic field lines of the earth. They are energetic enough to ionize air molecules, so a considerable number of atoms and molecules are elevated to excited states. When they make the transition back to their ground states they emit light characteristic of the atoms and molecules. Red and green light emitted from oxygen atoms is a constituent of the light seen at the poles. Atmospheric nitrogen also plays a role. An example of the colors that might be visible can be found by observing the nitrogen spectrum. Near the north pole the light show is called the aurora borealis and near the south pole it is called aurora australis.
|A polar satellite captured images of aurora over the South Pole of the Earth. UV photographs of Jupiter indicate that auroral phenomena occur in its polar regions. Images of Saturn aurora show a very active pulsating pattern.|
|This sketch of charged particles spiraling around magnetic field lines is conceptual only.|