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magnetic stormsSee geomagnetic storms.
strong disturbances of the earth’s magnetic field that severely disrupt the smooth diurnal course of the components of geomagnetism. Magnetic storms last several hours to several days and are observed simultaneously over the entire earth. They are manifested with greatest intensity (up to about 5 ×10-2 oersted) in the high latitudes. In the middle latitudes changes in the intensity of the geomagnetic field during a magnetic storm range from 0.1 to about 1 ampere per meter (A/m), or about 1 ×103-1 ×102 oersted.
Magnetic storms usually have preliminary, initial, and main phases, as well as recovery phases. During the preliminary phase, slight changes in the geomagnetic field are observed (mainly in the high latitudes) and typical short-period fluctuations of the field are excited. The initial phase is characterized by a sudden change in individual components of the field over the entire earth; the main phase, by large fluctuations in the field and a strong decrease in the horizontal component. In the restoration phase of a magnetic storm the field returns to its normal value. In a disturbed geomagnetic field a distinction is usually made among aperiodic variations, polar magnetic substorms (which appear in the middle latitudes in the form of coil-shaped disturbances), and specific short-period oscillations.
Magnetic storms are caused by streams of solar plasma from active areas of the sun that are superimposed on the calm solar wind. Therefore, they are more frequently observed near the maximums of the 11-year cycle of solar activity. When they reach the earth, the streams of solar plasma increase the compression of the magnetosphere, bringing about the initial phase of a magnetic storm, and partially penetrate the earth’s magnetosphere. The incidence of high-energy particles in the earth’s upper atmosphere and their effect on the magnetosphere cause the generation and intensification of electric currents that attain great intensity in the polar areas of the ionosphere, which is the reason for the high-latitude zone of magnetic activity. Changes in the magnetosphere-ionosphere current systems are manifested on the earth’s surface in the form of irregular magnetic disturbances.
The solar protons with energy levels from 1.6 ×1016 to 1.6 ×1014 joule (from 1 to 100 kilo electron volts) incident on the Van Allen radiation belt during a magnetic storm create an equatorial current ring at a distance of three to six earth radii from its center. The magnetic field of the ring weakens the geomagnetic field in the main phase of a magnetic storm. The breakup of the ring current as a result of collisions between protons and the neutral atoms of the hydrogen in the earth’s atmosphere and the occurrence of instabilities in the plasma lead to exponential attenuation of the magnetic field of the current during the restoration phase of the magnetic storm.
Magnetic storms are one of the main manifestations of a more general geophysical process, the magnetospheric storm. It is associated with the upper atmosphere occurrence of auroras, ionospheric disturbances, and X-ray and low-frequency emissions. During a magnetic storm there are considerable changes in the parameters of the ionospheric layers that reflect and absorb radio waves (height of their location, concentration of electrons, and so on), resulting in considerable interference with shortwave radio communication. In addition, during magnetic disturbances the upper atmosphere heats up and the warmth passes downward, into the troposphere, which promotes the development of circulatory motion and the formation of cyclones.
REFERENCESlanovskii, B. M. Zemnoi magnetizm [3rd ed.], vol. 1. Leningrad, 1964.
Akasofu, S.-I. Poliarnye i magnitosfernye subburi. Moscow, 1971. (Translated from English.)
A. D. SHEVNIN