Magnetic Variations

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Variations, Magnetic


continuous changes of the earth’s magnetic field with time. Magnetic variations are characterized by the deviation of the components of the geomagnetic field (the horizontal component H , the vertical component Z , and magnetic declination D) from the average values at the place of observation. In equatorial regions the average value of the total intensity of the earth’s magnetic field is 0.42 oersteds; it increases toward the poles and attains 0.70 oersteds. Instruments that measure the variations of H, Z , and D are called magnetic variometers. The magnitude and form of magnetic variations depends on the latitude of the place of observation, the time of year, and solar activity. Processing the results of measurements makes it possible to distinguish periodic and nonperiodic changes of cosmic, magnetospheric-ionospheric, and intraterrestrial origin in the geomagnetic field. (See Figure 1.)

Figure 1. Photographic record of the components of the earth’s magnetic field (Voeikovo Observatory, February 1969); solar-dirurnal, short-period irregular, and bay-shaped (1700-1800 hrs) magnetic variations are superimposed on the diurnal course of the components H , D , and Z . The arrows indicate the scale and direction of the readings of H , D , Z.

The principal cosmic source of disturbances in the geomagnetic field is the sun. Eleven-year magnetic variations, which are associated with cyclical variation in solar activity and comprise (for the total geomagnetic field intensity) 1 x 10-5 to 2 x 10-4 oersteds, have been shown. Periodic annual magnetic variations (5 x 10-5 to 3 x 10-4 oersteds) and solar-diurnal variations (up to 7 x 10-4 oersteds), caused by changes in the conditions of illumination of the earth during its orbital movement and rotation around its axis, also exist. The most intense magnetic variations occur under the influence of the solar wind, a current of charged particles coming from the sun. These particles—creating a system of electrical currents on the boundary of the earth’s magnetosphere, in its outer radiation belt, and in the ionosphere—cause magnetic storms (with amplitudes up to 5 x 10-2 oersteds) during which irregular (1 x 10-5 to 4 x 10-2 oersteds), disturbed solar-diurnal (1 x 10-4 to 4 x 10-3 oersteds), aperiodic (1 x 10-4 to 2 x 10-3 oersteds), and bay-shaped (3 x 10-4 to 4 x 10-3 oersteds) magnetic variations are observed. Resonance fluctuations of the magnetosphere under the influence of the solar wind are manifested in the form of stable, short-period magnetic variations 1 x 10-7 to 3 x 10-4 oersteds). Short-period irregular magnetic variations arise as a result of intrusion of charged particles in the ionosphere. Another example of magnetic variations of cosmic origin are the periodic lunar-diurnal variations of about (1-5) x 10-5 oersteds. Slower magnetic variations with durations of hundreds of years or more, known as centenary magnetic variations (10-4 to 10-3 oersteds), are apparently caused by slow movements of matter in the deeper layers of the earth. The study of magnetic variations aids in the discovery of the nature of terrestrial magnetism and the physical connections of the earth and the sun.