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The Earth–ionosphere waveguide〔Spies, Kenneth P. and James R. Wait, ''Mode calculations for VLF propagation in the earth-ionosphere waveguide'' (July 1961). U.S. National Bureau of Standards. QC100 .U5753 no.114 1961.〕 refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere. Because the ionosphere contains charged particles, it can behave as a conductor. The earth operates as a ground plane, and the resulting cavity behaves as a large waveguide. Extremely low frequency (ELF) (< 3 kHz) and very low frequency (VLF) (3–30 kHz) signals can propagate efficiently in this waveguide. For instance, lightning strikes launch a signal called radio atmospherics, which can travel many thousands of miles, because they are confined between the Earth and the ionosphere. The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7 Hz. ==Introduction== Radio propagation within the ionosphere depends on frequency, angle of incidence, time of day, season, Earth's magnetic field, and solar activity. At vertical incidence, waves with frequencies larger than the electron plasma frequency (fe) of the F-layer maximum (1) fe = 9 (Ne)1/2 kHz (Ne in cm−3 is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than fe are reflected within the ionospheric D-, E-, and F-layers.〔Davies, K., "Ionospheric Radio", Peregrinus Ltd, London, 1990〕〔Rawer, K., "Wave Propagation in the Ionosphere", Kluwer Publ., Dordrecht, 1993〕 fe is of the order of 8–15 MHz during day time conditions. For oblique incidence, the critical frequency becomes larger. Very low frequencies (VLF: 3–30 kHz), and extremely low frequencies (ELF: <3 kHz) are reflected at the ionospheric D- and lower E-layer. An exception is whistler propagation of lightning signals along the geomagnetic field lines.〔〔 Originally published by Stanford University Press, Stanford, California (1965).〕 The wavelengths of VLF waves (10–100 km) are already comparable with the height of the ionospheric D-layer (about 70 km during the day, and 90 km during the night). Therefore, ray theory is only applicable for propagation over short distances, while mode theory must be used for larger distances. The region between Earth's surface and the ionospheric D-layer behaves thus like a waveguide for VLF- and ELF-waves. In the presence of the ionospheric plasma and the geomagnetic field, electromagnetic waves exist for frequencies which are larger than the gyrofrequency of the ions (about 1 Hz). Waves with frequencies smaller than the gyrofrequency are called hydromagnetic waves. The geomagnetic pulsations with periods of seconds to minutes as well as Alfvén waves belong to that type of waves. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earth–ionosphere waveguide」の詳細全文を読む スポンサード リンク
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