By raising organisms in an environment with very little magnetic activity, researchers may be able to discern whether use of such fields is a learned or an innate trait. Any space with very little magnetic noise opens up the possibility for more detailed diagnoses: for instance, distinguishing the magnetic field of a mother's heart from that of her unborn child to determine irregularities.
Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. The force is weaker in the middle of the magnet and halfway between the pole and the center. If you were to sprinkle iron filings on a piece of paper and place the magnet beneath it, you could see the path of the magnetic field lines. The field lines are closely packed at either pole of the magnet, widening as they get farther from the pole and connecting to the opposite pole of the magnet.
The magnetic field lines emerge from the north pole and enter the south pole. The magnetic field gets weaker the farther you get from either pole, so a bar magnet is only useful for picking up small items over short distances. Iron filings make a pattern tracing field lines because each bit of iron is itself a tiny dipole the separation between magnetic fields.
The force the dipole experiences is proportional to the strength of the dipole and proportional to the rate at which the magnetic field changes. The dipole tries to align itself with a magnetic field, but at the ends of a bar magnet, the field lines are very close together. What this indicates is that the magnetic field varies strongly over a short distance compared to the variation closer to the middle of the magnet.
Because the magnetic field changes so dramatically, a dipole feels more force. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere.
At large distances from the planet, this dominates the surface magnetic field. Electric currents induced in the ionosphere also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations that can deflect surface magnetic fields by as much as one degree. Typical daily variations of field strength are about 25 nanoteslas nT i.
The currents in the core of the Earth that create its magnetic field started up at least 3, million years ago. Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artifacts , kilns, some types of stone structures, and even ditches and middens in archaeological geophysics. Using magnetic instruments adapted from airborne magnetic anomaly detectors developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped.
The basalt — the iron-rich, volcanic rock making up the ocean floor — contains a strongly magnetic mineral magnetite and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor.
When newly formed rock cools, such magnetic materials record the Earth's magnetic field. Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms , provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index.
Recently, leaks have been detected in the magnetic field, which interact with the Sun's solar wind in a manner opposite to the original hypothesis. During solar storms , this could result in large-scale blackouts and disruptions in artificial satellites. Based upon the study of lava flows of basalt throughout the world, it has been proposed that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years , with an average interval of approximately , years.
There is no clear theory as to how the geomagnetic reversals might have occurred. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact , and then restarts itself with the magnetic "North" pole pointing either North or South.
External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when the magnetic pole flips from one hemisphere to the other this is known as a reversal, whereas temporary dipole tilt variations that take the dipole axis across the equator and then back to the original polarity are known as excursions.
Studies of lava flows on Steens Mountain , Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day at some time in Earth's history, which significantly challenges the popular understanding of how the Earth's magnetic field works.
Paleomagnetic studies such as these typically consist of measurements of the remnant magnetization of igneous rock from volcanic events. Sediments laid on the ocean floor orient themselves with the local magnetic field, a signal that can be recorded as they solidify. Although deposits of igneous rock are mostly paramagnetic , they do contain traces of ferri - and antiferromagnetic materials in the form of ferrous oxides, thus giving them the ability to possess remnant magnetization.
In fact, this characteristic is quite common in numerous other types of rocks and sediments found throughout the world. One of the most common of these oxides found in natural rock deposits is magnetite. As an example of how this property of igneous rocks allows us to determine that the Earth's field has reversed in the past, consider measurements of magnetism across ocean ridges. Before magma exits the mantle through a fissure, it is at an extremely high temperature, above the Curie temperature of any ferrous oxide that it may contain.
The lava begins to cool and solidify once it enters the ocean, allowing these ferrous oxides to eventually regain their magnetic properties, specifically, the ability to hold a remnant magnetization. Assuming that the only magnetic field present at these locations is that associated with the Earth itself, this solidified rock becomes magnetized in the direction of the geomagnetic field. Although the strength of the field is rather weak and the iron content of typical rock samples is small, the relatively small remnant magnetization of the samples is well within the resolution of modern magnetometers.
The age and magnetization of solidified lava samples can then be measured to determine the orientation of the geomagnetic field during ancient eras. Governments sometimes operate units that specialise in measurement of the Earth 's magnetic field.
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