Helical fluid motions in Earth's electrically conducting liquid outer core have an electromagnetic dynamo effect, giving rise to the geomagnetic field. The planet's sizable, hot core, along with its rapid spin, probably accounts for the exceptional strength of the magnetic field of Earth compared with those of the other terrestrial planets.
Earth's main magnetic field permeates the planet and an enormous volume of space surrounding it. A great teardrop-shaped region of space called the magnetosphere is formed by the interaction of Earth's field with the solar wind. At a distance of about 65,000 km (40,000 miles) outward toward the Sun, the pressure of the solar wind is balanced by the geomagnetic field. This serves as an obstacle to the solar wind, and the flow of charged particles, or plasma, is deflected around Earth by the resulting bow shock. The magnetosphere so produced streams out into an elongated magnetotail that stretches several million kilometres downstream from Earth away from the Sun.
Plasma particles from the solar wind can leak through the magnetopause, the sunward boundary of the magnetosphere, and populate its interior; charged particles from Earth's ionosphere also enter the magnetosphere. The magnetotail can store for hours an enormous amount of energy—several billion megajoules, which is roughly equivalent to the yearly electricity production of many small countries. This occurs through a process called reconnection, in which the Sun's magnetic field, dragged into interplanetary space by the solar wind, becomes linked with the magnetic field in Earth's magnetosphere. The energy is released in dynamic structural reconfigurations of the magnetosphere, called geomagnetic substorms, which often result in the precipitation of energetic particles into the ionosphere, giving rise to fluorescing auroral displays.
Converging magnetic field lines fairly close to Earth can trap highly energetic particles so that they gyrate between the Northern and Southern Hemispheres and slowly drift longitudinally around the planet in two concentric doughnut-shaped zones known as the Van Allen radiation belts. Many of the charged particles trapped in these belts are produced when energetic cosmic rays strike Earth's upper atmosphere, producing neutrons that then decay into electrons, which are negatively charged, and protons, which are positively charged. Others come from the solar wind or Earth's atmosphere. The inner radiation belt was detected in 1958 by the American physicist James Van Allen and colleagues, using a GeigerMüller counter aboard the first U.S. satellite, Explorer 1; the outer belt was distinguished by other U.S. and Soviet spacecraft launched the same year. Earth's magnetosphere has been extensively studied ever since, and space physicists have extended their studies of plasma processes to the vicinities of comets and other planets.
An important characteristic of Earth's magnetic field is polarity reversal. In this process the direction of the dipole component reverses—i.e., the north magnetic pole becomes the south magnetic pole and vice versa. From studying the direction of magnetization of many rocks, geologists know that such reversals occur, without a discernible pattern, at intervals that range from tens of thousands of years to millions of years, though they are still uncertain about the mechanisms responsible. It is likely that during the changeover, which is believed to take a few thousand years, a nondipolar field remains, at a small fraction of the strength of the normal field. In the temporary absence of the dipole component, the solar wind would approach much closer to Earth, allowing particles that are normally deflected by the field or are trapped in its outer portions to reach the surface. The increase in particle radiation could lead to increased rates of genetic damage and thus of mutations or sterility in plants and animals, leading to the disappearance of some species. Scientists have looked for evidence of such changes in the fossil record at times of past field reversals, but the results have been inconclusive.
DEVELOPMENT OF EARTH'S STRUCTURE _AND COMPOSITION_
As the gas making up the solar nebula beyond the Sun cooled with time, mineral grains are thought to have condensed and aggregated to form the earliest mete-oritic material. In addition, as is suggested by the finding of anomalous concentrations of isotopes in a few meteorites, solid material from outside the solar system, apparently existing prior to the formation of the Sun, was occasionally incorporated into these developing small bodies.
The concentrations of isotopes that decay radioactively and of isotopes that are produced by radioactive decay provide scientists the information required to determine when meteorites and the planets formed. For example, the concentrations of rubidium-87 and the strontium-87 into which it decays, or those of samarium-147 and its decay product neodymium-143, indicate that the oldest meteorites formed some 4.56
billion years ago. Other isotope studies demonstrate that Earth formed within, at most, a few tens of millions of years after the birth of the Sun.
The most abundant elements in the Sun, hydrogen and helium, are severely lacking in the inner, terrestrial planets. When the relative abundances of the less volatile elements are compared for the Sun, for a class of primitive, largely chemically unaltered meteorites called CI carbonaceous chondrites (considered by many researchers the most pristine samples of original solar system material), and for the estimated composition of Earth, their values are all in close agreement. This is the basis for the chondritic model, which holds that Earth (and presumably the other terrestrial planets) was essentially built up from bodies made of such meteoritic material. This idea is corroborated by isotopic studies of rocks derived from interior regions of Earth considered to be little changed throughout the planet's history. Thus, it appears that the composition of Earth is roughly what would be expected given the observed elemental abundances in the Sun and accounting for the loss of the more volatile elements.
Stony meteorites and iron meteorites (those composed largely of iron alloyed with nickel and sulfur) both fall on Earth today, and both types are thought to have been present during the formation of the planetesimals that would accrete to become Earth. In other words, Earth seems to have accreted only after most, if not all, solid matter had already condensed. Thus, a wide range of minerals was included in the grains, the larger fragments, and even the planetesimals that were accumulated by the growing planet. Apparently, such an aggregation of dense metallic fragments and less dense rocky fragments is not very stable. Calculations based on the measured strengths of rocks indicate that the metallic fragments probably sank downward as Earth grew. Although the planet was relatively cold at this stage—less than 500 K (230 ° C, or 440 °F)—the rock was weak. This is an important point because it leads to the conclusion that Earth's metallic core began to form during accretion of the planet and probably before the planet had grown to one-fifth of its present volume.
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