Atmospheric interference, or atmospherics, is generated as a result of electromagnetic disturbances caused by electrostatic discharges between clouds, changes in ionization layers of the atmosphere under the influence of cosmic rays, electrification of the receiving antenna by moving charged dust or snow particles. According to other views, atmospheric interference originates from electrons emitted by the Sun, creating eddy currents in the Heaviside layer.
The intensity of such interference varies depending on frequency, time of day, geographic region, and weather. The energy of atmospheric impulse interference is mainly concentrated in the frequency range below 50 megahertz. Research by Austin and others has shown that the strength of atmospheric interference worldwide increases approximately in proportion to the wavelength. At frequencies above 50 MHz, weaker atmospheric interference occurs and, under normal conditions, is only weakly reflected by the ionosphere. Therefore, at frequencies above 50 megahertz, only weak interference of local origin occurs. It is practically absent when receiving on HF and VHF bands.
The most powerful source of atmospheric interference is the numerous lightning discharges occurring simultaneously in various regions of the globe. According to statistical data, about 2,000 thunderstorms occur simultaneously, with approximately 100 lightning strikes every second. Disregarding local thunderstorms, the level of atmospheric noise is quasi-stationary, depending on the geographical coordinates of the receiving point and changing relatively slowly over the course of a day and from season to season. This allows forecasting the level of atmospheric interference. Such forecasts are summarized in the special Report No. 322 of the Plenary Assembly of the International Radio Consultative Committee (CCIR).
The strongest and most frequent lightning discharges are observed in equatorial and tropical regions. Thus, the main mass of atmospheric interference reaches the Northern Hemisphere from the equator. The level of atmospheric interference decreases with increasing receiver tuning frequency and distance from the equator. The direction of atmospheric interference can be determined by standard technical direction finders, preferably unidirectional ones. Such research has shown, for example:
1. The source of atmospheric interference for North America lies in Mexico; for Western Europe – in the European part of Russia (especially in summer months);
2. There are daily and annual periodic changes in the direction of atmospheric interference. Most often in the morning hours, the main direction of atmospheric interference lies in the north–south plane, while around noon this direction shifts eastward;
3. In summer, atmospheric interference almost always has a clearly defined direction;
4. There is an entire class of atmospheric interference without a specific direction.
At significant altitudes above the Earth's surface, such as in mountainous areas, the level of atmospheric interference increases. Near seas, the interference level is much lower than in the central parts of continents (for example, for critical reception from America in Berlin, it was found most advantageous to set up a receiving radio station 400 km from Berlin on the seashore). In mountainous regions, atmospheric interference intensifies. In summer, the overall background of atmospheric interference is significantly higher due to increased solar activity.
Atmospheric interference in the form of smooth white noise, whose intensity is almost independent of frequency and whose spectral density is evenly distributed, originates from fluctuations in the Earth's electric field, quiet atmospheric discharges, auroras, and radiation from cosmic objects. This interference has no preferred arrival directions, and its sources can be considered evenly distributed over the entire sphere.
Various electrical processes constantly occur in the Earth's atmosphere, such as cloud electrification and auroras in polar regions. The connection between atmospheric interference and the aurora phenomenon is not yet sufficiently understood. During solar eclipses, a reduction in atmospheric interference is always observed with the onset of darkness, followed by a gradual return to previous levels after the eclipse ends.
Electric currents arise in the ionized layers of the atmosphere (atmospheric electricity). These phenomena create electromagnetic fields that, propagating through space and reaching receiving antennas, induce alternating currents of various frequencies, resulting in crackling sounds in radio receivers' headphones and loudspeakers. Interference at the receiver input caused by antenna canvas electrification manifests as prolonged noise or whistling. Under normal conditions, the level of such interference is low.
No clear dependence of atmospheric interference on meteorological factors (cloud cover, humidity, wind speed, etc.) has yet been established. It is only known that strong atmospheric interference appears during barometric depressions: atmospheric interference coincides in direction with the azimuth of areas experiencing low pressure at that time. In 1927, in America, ships began using direction finders to determine the direction of atmospheric interference in order to track cyclone movements to some extent. In all cases of sudden weather changes, an increase in atmospheric interference is noted.
Atmospheric interference was first discovered by the great radio inventor A. S. Popov, who in 1895 demonstrated the first radio receiver in the world, and in the summer of 1896 built the globally famous lightning detector, which recorded changes in the electric field caused by thunderstorms. The study of the discharge process of atmospheric electricity began only in recent years, with observations carried out using Braun tubes and specialized electronic oscillographs. As a result of observations, two groups of atmospheric discharges were identified: a) aperiodic – with an average duration of 3.1*10-3 seconds; and b) periodic – with an average duration of 1.9*10-3 seconds.
Related sections:
Static interference,
Industrial interference,
Interference phenomenon,
Ionosphere, ionospheric radio waves.