Solar-Terrestrial Relationships

the reaction of the earth—that is, of its outer envelopes, including the biosphere—to changes in solar activity. The level of solar activity—for example, the number of active regions, the number of sunspots, and the number and size of solar flares—varies with a period of approximately 11 years. There also exists a low-amplitude variation, with a period of approximately 80 years, in the magnitude of the maximums of the 11-year cycle. On the earth, the 11-year cycle can be traced in a number of phenomena in organic and inorganic nature, such as magnetic disturbances, auroras, ionospheric disturbances, and the growth rate of trees (as evidenced by an alternation in tree-ring thicknesses with an 11-year period).

Terrestrial processes are also influenced by individual active regions on the sun and by the short-lived but sometimes very energetic flares that occur in these regions. An individual active region on the sun may last as long as a year. The disturbances caused by the active region in the magnetosphere and upper atmosphere of the earth recur at 27-day intervals, which correspond to the rotation period of the sun as seen from the earth.

Solar flares are the most striking and violent manifestations of solar activity. They occur irregularly but are particularly often associated with a maximum in solar activity. They generally last 5–10 min but occasionally persist as long as several hours. The energy released in a flare can be as great as ~10³² ergs (~10²⁵joules). Only 1–10 percent of the energy is emitted as electromagnetic radiation in the optical region of the spectrum. The energy of a flare constitutes a very small part, ~10~⁵—10^-6, of the total solar emission in the optical region. A substantial contribution, however, may be made to the X-radiation and corpuscular radiation of the sun by the short-wavelength radiation of a flare, the fast electrons produced in a flare, and, sometimes, the solar cosmic rays associated with a flare.

In periods of increasing solar activity, the X-ray emission of the sun increases in the 30–10 nanometer (nm) range by a factor of two, in the 10–1 nm range by a factor of three to five, and in the 1–0.2 nm range by a factor of more than 100. The shorter the wavelength of the radiation, the greater the contribution of the active regions to the total solar emission. In the 1–0.2 nm range, practically all the radiation is due to the active regions. Hard X-radiation with wavelength λ < 0.2 nm appears in the solar spectrum for only a brief period after flares.

In the ultraviolet region (λ from 180 to 350 nm), solar emission varies by only 1–10 percent during the 11-year cycle. In the 290–2,400 nm range, solar emission remains practically constant and has been calculated to be 3.6 x 10³³ ergs/sec, or 3.6 × 10²⁶watts.

The constancy of the energy received by the earth from the sun (see) ensures the stability of the heat balance of the earth. Solar activity does not have a significant effect on the energy system of the earth as a whole, but individual components of the radiation from flares and active regions can have a considerable influence on many physical, biophysical, and biochemical processes on the earth.

Active regions are an important source of corpuscular radiation. The solar wind—that is, the flux of particles continuously emitted by the sun—is intensified by particles, primarily protons, with energies of approximately 1 kiloelectron volt (keV) that travel from the active regions along the lines of force of the interplanetary magnetic field. Such high-speed streams in the solar wind often recur at intervals of 27 days and are said to be recurrent. Similar streams, of still greater energy and density, arise in association with flares. Such streams are responsible for sporadic disturbances of the solar wind and take between eight to ten hours and two days to reach the earth. High-energy protons (from 100 million electron volts to 1 gigaelectron volt) from very intense “proton” flares and electrons with energies of 10–500 keV reach the earth as solar cosmic rays in tens of minutes after the onset of the flares. Protons and electrons that are trapped in the interplanetary magnetic field and move along with the solar wind arrive at the earth somewhat later. The short-wavelength radiation and solar cosmic rays (at high latitudes) ionize the earth’s atmosphere. As a result, fluctuations occur in atmospheric transmissivity in the ultraviolet and infrared regions. An additional consequence is a variation in shortwave propagation conditions; in a number of cases, interruption of shortwave radio communication is observed (seeIONOSPHERE).

The solar-wind enhancement caused by a flare has a number of effects, for example, a compression of the earth’s magnetosphere on the sunward side, an intensification of the currents at the outer boundary of the magnetosphere, a partial penetration of solar-wind particles deep into the magnetosphere (into the zone of auroral radiation), and the addition of high-energy particles to the earth’s radiation belts (see). These processes are accompanied by fluctuations in the strength of the geomagnetic field (magnetic storms), by auroras, and by other geophysical phenomena reflecting the general disturbance of the earth’s magnetic field (seeVARIATIONS, MAGNETIC).

Thus, active processes on the sun exert an influence on geophysical phenomenona both by means of short-wavelength radiation and through the earth’s magnetic field. Short-wavelength radiation and the earth’s magnetic field apparently are the principal intermediaries for physicochemical and biological processes as well (seeMAGNETOBIOLOGY). It is not yet possible to trace the entire chain of relationships leading to the 11-year periodicity of many processes on the earth, but the extensive data that have been accumulated leave no doubt concerning the existence of such relationships. For example, a correlation has been found between the 11-year cycle of solar activity and earthquakes, lake-level fluctuations, crop yields, the reproduction and migration of insects, epidemics of influenza, typhus, and cholera, and the incidence of cardiovascular diseases. These data indicate the existence of continuously acting solar-terrestial relationships.

The discovery of the mechanisms of solar-terrestrial relationships is of great scientific and practical interest. In particular, such knowledge will permit substantial improvements in the accuracy of long-range weather forecasts and in the prediction of the intensity of particle fluxes in space near the earth (such predictions are essential for spaceflight).

The effect of solar-terrestial relationships on physical processes is studied by heliogeophysics, the effect on biological processes is studied by heliobiology, and the effect on the weather is studied by heliometeorology.