geomagnetism: external fields

geomagnetism: external fields Not only does the Earth have its self-generated internal magnetic field (see magnetic field, origin of the internal field); it is also surrounded by magnetic fields formed outside the Earth. These fields are called external fields. External fields originate as a result of complex conditions created in Earth's vicinity by the interactions of solar wind, solar radiation, the Earth's internal field, and the Earth's atmosphere. Extremely high temperatures prevail in the Sun's corona, a region immediately above the surface of the Sun. In the corona, protons and electrons of solar hydrogen can exist separately as charged particles in an ionized state known as plasma. Because of their very high speeds, these charged particles defy the Sun's gravitational field and continuously break away from the corona in a radially outward manner. The constant bombardment of the charged particles into space, with velocities of some 400 km/s is called solar wind. The solar wind is deflected around obstacles in its path—obstacles such as planets, comets, and other objects in the solar system. (The first clues to the existence of the solar wind were in fact provided by the tails of comets, which always point away from the Sun.) But because the solar wind is made up of charged particles, the form of the internally generated magnetic field of a planet is significantly distorted by the solar wind. The interaction between the two encloses and compresses the planetary magnetic field on the side of the Sun (in the planet–Sun axis) and drags it further out on the side away from the Sun. This vast region in which the geomagnetic field is enclosed is called a magnetosphere (Fig. 1). Each planet that has its own internally generated magnetic field has its own magnetosphere. Within the magnetosphere, trapped charged particles (in a region known as Van Allen belts) result in one of the most prominent magnetospheric currents, the ring current. Its magnetic effect in the near-Earth environment can be large, measured in hundreds of nanoteslas (nT) (see geomagnetic measurement: techniques and surveys).

Planets with atmospheres have regions surrounding them called ionospheres. The presence of the Earth's ionosphere makes worldwide short-wave communications possible because the radio waves are bounced off this region. The ionosphere is produced by bombardment of solar ultraviolet photons on the atmosphere. The photons can break apart, or ionize, molecules and atoms of the atmosphere into protons and electrons, producing plasma. The ionized particles of the plasma are significantly affected by the Earth's magnetic field and the atmospheric circulations caused by solar heat and the Earth's rotation, which cause them to flow in various forms of circulation patterns (in the bulk sense). In 1882, Stewart suggested that the flow of these charged particles creates electrical currents and, consequently, the magnetic fields in the ionosphere (see magnetic field (origin of the Earth's internal field)). During normal solar activity, two magnetic effects from currents in the ionosphere are prominent in the equatorial and mid-latitude regions. One is called Sq (solar quiet) and the other is equatorial electrojet (Fig. 2); both are strongest during a local noontime. The Sq current has its centres near 30° magnetic (dip) latitude (see geomagnetic measurement: techniques and surveys) in both hemisphere, flowing counterclockwise in the northern and clockwise in the southern hemisphere. Its magnetic effects range up to about 40 nT near the Earth's surface. The equatorial electroject, on the other hand, flows eastward during the daytime in a narrow swath of latitudes along the magnetic (dip) equator; it is also restricted in altitude, close to about 105 km above the Earth. Its magnetic effects at the Earth's surface range up to about 100 nT.

Clinging tightly to the superhighway of magnetic field lines, the charged particles in the solar wind are accelerated by the Earth's magnetic field, attaining speeds of over 60 000 km/s during solar storms. Their descent into the Earth's polar ionosphere is marked by colourful shimmering curtains of light called the aurora. Aurorae are by far the most spectacular of geomagnetic phenomena. When the charged particles of the solar wind hit the atmosphere, various gases present in the atmosphere heat up, split apart molecules, and ionize atoms. Eventually, the atoms cool down, deionize (i.e. gain some electrons back), and in the process emit photons (in a process almost opposite to the one that causes ionization in the equatorial region). According to their energy, the charged particles penetrate to different levels in the atmosphere, ionize different gases, and hence produce the different colours of the aurorae. In addition to aurorae, complex interactions between the magnetosphere and the ionosphere take place over the polar regions. Streams of charged particles produce field-aligned (or Birkeland) currents. Coupling between the ionosphere and the atmospheric circulation around the poles also produces an intense current called the auroral electrojet, with magnetic effects at the Earth's surface as high as 1000 nT.

D. Ravat

Bibliography

Consolmagno, G. J. and and Schaefer, M. W. (1994) Worlds apart: a textbook in planetary sciences. Prentice Hall, New Jersey.