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the electrical signals created by radio waves radiated from lightning discharges. Approximately 100 lightning discharges per second occur near the earth’s surface. Consequently, atmospherics can be recorded almost continuously at any point on the globe. In radio reception, atmospherics are heard as a rustling noise or characteristic whistles which create atmospheric interference with radio reception. A lightning discharge has two stages—a leader stroke and a main stroke—which differ in current strength and the spectrum of the radio waves emitted (see Figure 1). The main stroke radiates superlong waves and the
leader stroke radiates long waves, medium waves, and even short waves. The maximum energy of atmospherics lies in a frequency range on the order of 4–8 kilohertz. If they are produced by local thunderstorms, their spectrum then depends only on the radiation spectrum of the lightning discharge. If, however, the source is a distant storm, the spectrum also depends on the radio wave propagation conditions from the locus of the thunderstorm to the radio receiving apparatus.
Some atmospherics are heard as signals having a frequency that decreases continuously; they are known as whistling atmospherics. Their characteristics are due to the mechanism of superlong-wave propagation. As these waves propagate in the waveguide formed by the lower boundary of the ionosphere and the earth’s surface, a partial “leakage” through the ionosphere occurs. The waves which have leaked out, propagating along the lines of force of the earth’s magnetic field, move tens of thousands of kilometers away from the surface of the earth and then return again. Their propagation velocity is a function of their frequency; the high-frequency signal components are propagated with greater velocity and arrive earlier. This results in the generation of the characteristic whistle with continuously changing pitch at the receiver output.
Research on atmospherics provides information on the propagation mechanism of superlong waves and also on the very lowest and highest regions of the ionosphere in which the atmospherics propagate. In order to calculate radio communication links, special charts and nomograms are constructed from which the level of atmospherics can be determined at any point of the earth.
REFERENCESAl’pert, Ia. L. Rasprostranenie radiovoln i ionosfera. Moscow, 1960.
Dolukhanov, M. P. Rasprostranenie radiovoln, 2nd ed. Moscow, 1960.
Krasnushkin, P. E. “Atmosferiki.” In Fizicheskii entsiklopedicheskii slovar’, vol. 1. Moscow, 1960. Pages 100–102.
M. B. VINOGRADOVA