sferics

1. The study of atmospherics, especially through the use of electronic detectors.
2. See atmospherics (sense 1).

[Shortening and alteration of ATMOSPHERICS.]

Electromagnetic radiations produced primarily by lightning strokes from thunderstorms. It is estimated that globally there occur about 2000 thunderstorms at any one time, and that these give rise to about 100 lightning strokes every second. The radiations are short impulses that usually last a few milliseconds, with a frequency content ranging from the low audio well into the gigahertz range. Sferics (short for atmospherics) are easily detected with an ordinary amplitude-modulation (AM) radio tuned to a region between radio stations, especially if there are thunderstorms within a few hundred miles. These sounds or noises have been identified and characterized with specific names, for example, hiss, pop, click, whistler, and dawn chorus. They fall into what is generally known as radio noise. See also Atmospheric electricity; Dust storm; Electromagnetic radiation; Lightning.

The various types of sferics include terrestrial, magnetospheric, and Earth-ionospheric. Terrestrial sferics includes anthropogenic noise from sources such as automobile ignition, motor brushes, coronas from high-voltage transmission lines, and various high-current switching devices. Dust storms and dust devils have also been observed to produce sferics. See also Electrical noise.

Lightning-generated sferics are sometimes coupled into the magnetosphere, where they are trapped and guided by the Earth's magnetic field. In this mode, the impulse travels in an ionized region. As a result, the frequencies present in the original impulse are separated by dispersion (the higher frequencies travel faster than the lower) and produce the phenomena known as whistlers. See also Magnetosphere.

By far the dominant and most readily observed sferics are the lightning-produced impulses that travel in the spherical cavity formed by the ionosphere and the Earth's surface. Lightning currents produce strong radiation in the very low-frequency band, 3–30 kHz, and in the extremely low-frequency band, 6 Hz–3 kHz. See also Ionosphere; Radio-wave propagation.