Radio Radiation, Solar

electromagnetic radiation from the solar atmosphere in the wavelength band from fractions of a millimeter to several kilometers. Solar radio radiation was first detected in the mid-1930’s, when the existence of radio interference, whose intensity varied with solar activity, was discovered. In 1942, in addition to this type of solar radio radiation, referred to as radio radiation from the active sun, radio radiation from the quiet sun in the decimeter band was also detected. Systematic studies of solar radio radiation were begun in the period 1946–47.

Solar radio radiation at wavelengths of approximately 1 mm to tens of meters is studied from the earth with radio telescopes; spacecraft are used in studying radio radiation at longer and shorter wavelengths. Solar radio radiation at wavelengths greater than several kilometers is nearly entirely absorbed in the interplanetary gas and is inaccessible to observations.

Radio radiation from the quiet sun is nearly constant with time and is associated with the thermal emission of electrons into the electric ion field of an undisturbed solar atmosphere. Shortwave radio radiation (1–3 mm) emanates from the sun’s photosphere, radiation at centimeter wavelengths originates in the chromosphere, and radiation at decimeter and meter wavelengths emits from the corona, which extends great distances from the visible solar disk and blends continuously into the interplanetary gas. In 1947, Soviet scientists were the first to establish that radiation in the meter wavelength band originated in the corona of the quiet sun; this was done during observations of a total solar eclipse. At the same time it was also discovered that the temperature of the corona is approximately 10⁶ °K

Solar radio radiation that varies slowly is chiefly associated with active regions in the solar atmosphere above sunspots, as well as with plages; the radiation is also thermal in nature. However, magnetobraking radiation apparently plays a role here, in addition to bremsstrahlung; that is, the radiation in part results from the bending of electron paths by the magnetic fields of sunspots. This type of solar radio radiation predominates in the 5–20 cm wavelength band and corresponds to periods of solar activity visible in the optical band; in particular, it corresponds to sunspot areas. Such radio radiation often exhibits strong circular polarization, which indicates the presence of powerful (up to several thousand oersteds) magnetic fields in the areas responsible for the radiation.

Bursts of solar radio radiation are highly diverse, sometimes millions of times greater than the thermal radio radiation from the quiet sun. This type of solar radio radiation predominates in the meter wavelength band, although impulsive bursts have even been detected at millimeter wavelengths. Relativistic particles are emitted during flares near sunspots. Their motion through the solar atmosphere gives rise to powerful radio radiation. The radio radiation involves either magnetobraking radiation or excitation of different waves in the solar plasma followed by transformation of plasma waves into electromagnetic waves. Small quasi-periodic fluctuations in solar radio radiation with periods of hundreds and thousands of seconds and extremely low amplitudes have also been detected. The nature of such fluctuations has not yet been explained (1975).

Results of observations of solar radio radiation are used to construct models of the solar atmosphere and to study the mechanism by which the sun affects the earth’s atmosphere. The study of the sun by means of radar constitutes the subject of radar astronomy.