Magnetic Field

The scientific community is fairly well united in the idea that the con-vective currents in the outer core combined with the planet's rotation cause the Earth's magnetic field. Sir Joseph Larmour, an Irish physicist and mathematician, first proposed the hypothesis that the Earth's active magnetic field might be explained by the way the moving fluid iron in Earth's outer core mimics electrical currents, and the fact that every electric current has an associated, enveloping magnetic field.The combination of convective currents and a spinning inner and outer core is called the dynamo effect. If the fluid motion is fast, large, and conductive enough, then a magnetic field can not only be created, but also carried and deformed by the moving fluid (this is also what happens in the Sun). The inner core rotates faster than the outer core, pulling field lines into itself and twisting them. Fluid upwelling from the boundary with the inner core is twisted by the Coriolis effect, and in turn twists the magnetic field. In these ways, it is thought that the field lines are made extremely complex.The exact patterns of the field at the core are not known.This theory works well for the Earth and for the Sun, and even for Jupiter and Saturn, though they are thought to have a dynamo made of metallic hydrogen, not iron.

The Earth's magnetic field is largely a bipolar field, meaning that it has a North and a South Pole with magnetic field lines that flow between them (see figure below). Magnetic fields are often thought of

Dipolar and Quadrupolar Field Lines

Magnetic and rotational axis

Magnetic and rotational axis

A dipolar planetary magnetic field resembles the field of a bar magnet.

just in terms of dipoles, meaning a two-poled system like a bar magnet, with magnetic field lines flowing out of the south magnetic pole and into the north magnetic pole, but there are other, more complex configurations possible for magnetic fields. The next most complex after the dipole is the quadrupole, in which the field has four poles equally spaced around the sphere of the planet. After the quadrupole comes the octupole, which has eight poles. Although the Earth's field is largely a dipole field, it has fine, weak structure related to quadru-pole, octupole, and higher order fields present as well. The dipole field is the strongest, and the strength of the higher fields falls off by nine orders of magnitude as the field complexity moves from quadru-pole up through the next ten levels of field complexity.

The word poles was first used for the magnetic field in 1269 by a man whose scholarly name was Petrius Peregrinus (actually Pierre de Maricourt of France). Because the magnetic field points straight into the North Pole of the Earth and straight out the South Pole, the magnetic field is also automatically aligned with the celestial poles, the imaginary points in the sky directly above the north and South Poles of the Earth. Peregrinus believed that the magnetic field was caused by something extraterrestrial, and so named the high-flux points of the magnetic field poles.

In 1600 William Gilbert of England was the first researcher who wrote that the Earth itself was the giant magnet. Finally, in 1838, the great mathematician C. F. Gauss developed the mathematics governing the magnetic field. Gauss also tried, without success, to understand the slow change in the magnetic field called secular variation that had been known to navigators for over 200 years. The term secular variation means change over time. The direction and intensity of the Earth's magnetic field does change over many years (on the Internet there are gorgeous animations of the field changing over hundreds of years). At the moment, the north magnetic pole of the Earth is changing location at about 25 miles per year (40 km/yr).The secular variation of the field reveals complex magnetic configurations underlying the basic dipole structure.The dominant dipole pattern is especially apparent when magnetic field values are averaged over a 10,000-year period.

In general, the Earth's field is strongest near the poles and weakest near the equator, though this generalization can be changed by secular variations in the field.The average strength of the Earth's field at the equator is about 30,300 nT (nanoteslas), stronger than the surface magnetic fields of any other planet except Jupiter, whose surface field is an immense 428,000 nT. Saturn, Uranus, and Neptune are close to the value of Earth, at 21,800, 22,800, and 13,300 nT, respectively. Mercury has a tiny field, at 300 nT, and Venus, the Moon, and Mars all have surface fields below 60 nT. The strength of the Earth's field sustains life on Earth by shielding us from radiation, but the strength of the field compared to those ofVenus and Mars, is somewhat of a mystery. Why have those planets' internal dynamos ceased, while ours continues? The Earth's magnetic field actually completely reverses at intervals varying from about 10,000 to 100,000 years, putting magnetic north at the South Pole, and vice versa. By identifying which magnetic interval during which a rock was formed can even be used to date rocks in a relative sense. And the fact that the magnetic field reverses was first discovered by investigating patterns of remanent magnetism (magnetism frozen into the rock) in oceanic crust.

At mid-ocean rises, magma melts in the mantle and rises in sheets and tubes to form new oceanic crust as it cools. As the magma cools to form new oceanic crust, a few percent of the minerals that form are magnetites or other oxides that can hold a magnetic field. These tiny grains are aligned by the Earth's magnetic field as they cool, and when they have cooled past a certain temperature, the magnetic field that they were formed in is permanently locked into the rocks, and can be detected by instruments called magnetometers. Reversals of the magnetic field were first measured in patterns of magnetic rocks in the oceanic crust, and through this study, plate tectonics was first proven. The magnetic field recorded in oceanic crust produced at mid-ocean ridges has alternating bands in which the magnetic field is first in the same direction as it is today, and then in the opposite direction, back and forth in stripes of variable width across the entirety of all the oceanic basins.

The Earth's magnetic field can be measured with great precision, and currently the dipole field is weakening. It has been weakening for hundred of years, and it is weakening faster and faster. In the last 150 years the field has lost about 10 percent of its strength.This weakening shows, it is thought, that the field is preparing to reverse and may do so within about a thousand years. Complex computer models have been made to try to understand magnetic reversals, and it is thought that as a reversal approaches magnetic field strength dwindles by

The aurora australis is caused by charged particles from the solar wind spinning along the Earth's magnetic field and bombarding atmospheric particles, which then emit light. The aurora in the Northern Hemisphere is called the aurora borealis. (National Oceanic and Atmospheric Administration/ Department of Commerce)

orders of magnitude and also becomes much more complex. As the field weakens, it degenerates into quadrupole and octupole fields as it reverses, and then reforms into the reversed dipole field. A magnetic reversal will have an immense effect on life on Earth. The magnetic field protects us from the most dangerous kinds of solar and interstellar radiation. As the field weakens, more radiation will reach the Earth, cancer rates will soar, and some extinctions will probably occur. The aurora australis (seen in the Southern Hemisphere and shown above) and borealis (Northern Hemisphere) are bands and streaks of light caused by the high-energy particles of the solar wind striking the Earth's ionosphere. Energy from these charged particles is converted into light, usually greenish, but occasionally red or orange. Because the particles in the solar wind are charged, they are attracted by the magnetic field lines of the Earth.The Earth's magnetic field is strongest near the poles, and so the majority of charged particles are gathered there, and the strongest auroras are formed in circles around the north and south magnetic poles, following the magnetic field lines. The strength of the solar wind determines the brightness of the aurora, and since the solar wind is variable, the brightness of the auroras is also variable.The stronger the solar wind, the more likely the auroras are to be visible at lower latitudes.

The 11-year cycle in sunspot population creates an 11-year cycle in the strength of the solar wind, and therefore a similar cycle in the general strength of auroras on Earth (and on other planets with magnetic fields, which experience similar phenomena).Though this is an overarching cycle, there are local disturbances during unusual solar storms that temporarily strengthen the solar wind. During one recent strong solar storm the aurora was strikingly bright as far south in the United States as Massachusetts, while usually it is only visible much closer to the Arctic circle. As a strange and interesting sideline to the discussion of auroras, astronauts orbiting within the Earth's ionosphere can also view auroras, but in an unusual way:The tiny charged particles pass through their bodies, without creating any sensation, except that when the astronaut close their eyes they can see tiny, bright flashes of light from the solar wind striking their optic nerves.

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