The geomagnetic field and magnetosphere
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. Venus, for example, which has a metallic core that may be similar to Earth’s in size, rotates very slowly and has no detected intrinsic magnetic field. Mercury and Mars have only small intrinsic magnetic fields.
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 the 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 smaller 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 Geiger-Müller 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. (For additional information on the interaction of the Sun and Earth’s charged particles and magnetic fields, see plasma: Solar-terrestrial forms.)
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.