Solar Radiation and Its Role in Plant Growth

THE SUN: THE SOURCE OF ENERGY

The sun is the nearest star to the earth, and its radiant energy is practically the only energy source to the earth. Very small and insignificant quantities of energy are available from other sources such as the interior of the earth, the moon, and other stars. The mean sun-earth distance, also known as one astronomical unit (1 AU), is 1.496 x 108 km or, more accurately, 149, 597, 890 ± 500 km. The earth revolves round the sun in an elliptical orbit. The minimum sun-earth distance is about 0.983 AU and the maximum approximately 1.017 AU. The earth is at its closest point to the sun (perihelion) on approximately January 3 and at its farthest point (aphelion) on approximately July 4. The visible disk or photosphere has a radius of 6.599 x 105 km, and the solar mass is 1.989 x 1030 kg (Goody and Yung, 1989; Iqbal, 1983).

The sun is a completely gaseous body. The chemical composition of the outer layers is (by mass) 71 percent hydrogen, 26.5 percent helium, and 2.5 percent heavier metals. Its physical structure is complex, although several regions, including the core, photosphere, reversing layer, chromosphere, and corona, are well recognized.

The innermost region, the core, is the densest and hottest part of the sun. It is composed of highly compressed gases at a density of 100 to 150 g-cm-3. The core temperature is in the range of 15 x 106 to 40 x 106°C. Outside the core is the interior which contains practically all of the sun's mass. The core and interior are thought to be a huge nuclear reactor in which fusion reactions take place. These reactions supply the energy radiated by the sun. The most important reaction is the process by which hydrogen is transformed to helium. The energy is first transferred to the surface of the sun and then radiated into space. The radiation from the core and interior of the sun is thought to be in the form of X rays and gamma rays.

The surface of the sun, called the photosphere, is the source of most of the visible radiation arriving at the earth's surface. The photosphere is the crust that is visible to the naked eye when looking at the sun through a blue glass. It is composed of very low density gases. The temperature in this region is 4,000 to 6,000oC. In spite of the fact that it has low density (10-4 that of air at sea level), the photosphere is opaque because it is composed of strongly ionized gases. The photosphere is the source of radiation flux to space because it has the capability to emit and absorb a continuous spectrum of radiation.

Outside the photosphere is the solar atmosphere, which is several hundred kilometers deep and almost transparent. This solar atmosphere is referred to as the reversing layer. This layer contains vapors of almost all of the known elements found on the earth. Outside the reversing layer is the chromosphere, which is about 25,000 km deep. It is seen from the earth only during a total eclipse when it appears as a rosy color layer. It is in this zone that the short-lived, brilliant solar flares occur in the clouds of hydrogen and helium. These flares are a source of intense bursts of ultraviolet (UV) and radio wave radiation. The solar flares also eject streams of electrically charged particles called corpuscles, which, on reaching the earth's surface, disturb its magnetic field. The temperature in the chromosphere is several times higher than that of the photosphere.

The outermost portion of the sun is the corona, which is composed of extremely rarefied gases known as the solar winds. These winds are believed to consist of very sparse ions and electrons moving at very high speeds and are thought to extend into the solar system. The corona can be seen during a total eclipse. It has a temperature on the order of 1,800,000°K. There is no sharp boundary to this outermost region.

These zones suggest that the sun does not act as a perfect black body radiator at a fixed temperature. The radiation flux is the composite result of its several layers. For general purposes, however, the sun can be referred to as a black body at a temperature of 5,762°K. The sun rotates at a rate that is variable in depth and latitude. As measured by the motion of sunspots, the synodic period (as seen from the earth) is 26.90 + 5.2 sin2 (latitude) days.

The sun is a variable star. It is estimated to be about 5 x 109 years old. Theories of climatic changes on geological time scales indicate definite changes that must have taken place during the lifetime of the sun. According to widely accepted theories, when the sun was formed it was 6 percent smaller and 300°K cooler, and its irradiance was 40 percent lower thanpres-ent-day values (Goody and Yung, 1989).

Some of the variations occurring in the sun are monitored on a regular basis. These variations are associated with magnetic activity resulting from interactions between convective motions, the solar rotation, and the general magnetic field of the sun. Magnetic fields and electric currents penetrate the chromosphere and corona, where magnetic variations have far greater influence because of the low densities.

The most striking visual disturbances are on the photosphere, and these are known as sunspots. These are patches varying in diameter from a few thousand to 100,000 kilometers, with an emission temperature in the center about 1,500°K lower than that of the undisturbed photosphere. The fraction of the photosphere covered by spots is never more than 0.2 percent, and their average persistence is about a week. For most of the period for which the observations are available, a sunspot cycle averages 11.04 years. The number of spots is only one characteristic feature of the sun that changes in this rhythmic manner. Just after the minimum, spots first appear near 27° latitude in both hemispheres. As the cycle proceeds, they drift equatorward and disappear close to 8° latitude. They are rarely observed at latitudes higher than 30° or lower than 5°.

When a sunspot is near to the extremity it can be seen to be surrounded by a network of enhanced photospheric emission, patches which are called faculae. These photospheric emissions have longer lifetimes than the associated sunspot group, appearing before and disappearing after the spots themselves.

Flocculi or plages are other disturbances that are typical features in hydrogen light (H-alpha). Flocculi are the most prominent features, and they occur at high latitudes, where spots do not. Occasionally, a hydrogen flocculus near a spot will brighten up. In extreme cases, the brightening is visible to the eye. These brightenings are known as solar flares, and they are associated with great increases of Lyman alpha and other ultraviolet radiations that influence the upper atmosphere.

Prominences are photospheric eruptions extending into the chromosphere. Many different forms occur, but a typical prominence might be 30,000 km high and 200,000 km long, with a temperature of 5,000°K.

Large changes in the corona are well established. Coronal ultraviolet emission is the heat source for levels in the upper atmosphere where the density is very low. The thermosphere, above 150 km, is greatly influenced by variable conditions on the sun. Coronal disturbances are closely related to the sunspot cycle. In visible light the corona appears more jagged at the sunspot maximum than at the minimum. Solar radio emission from the corona shows a marked variation with the sunspot cycle and is also correlated with shorter period changes in sunspot number.

Solar Constant

The sun is the source of more than 99 percent of the thermal energy required for the physical processes taking place in the earth-atmosphere sys tem. The solar constant is the flux of solar radiation at the outer boundary of the earth's atmosphere, received on a surface held perpendicular to the sun's direction at the mean distance between the sun and the earth. The value of the solar constant is 1,370 W m-2 (about 2 cal-cm-2-min-1), giving an average flux of solar energy per unit area of the earth's surface equal to 350 W m-2. The solar constant is only approximately constant. Depending on the distance of the earth from the sun, its value ranges from approximately 1,360 to 1,380 W m-2.

Of this energy, approximately 31 percent is scattered back to space, 43 percent is absorbed by the earth's surface, and the atmosphere absorbs 26 percent. The ratio of outward to inward flux of solar radiation from the entire earth's surface (termed albedo) is about 0.31, leaving an average around 225 W m-2 (range 220 to 235 W m-2) that is available for heating, directly and indirectly, the earth-atmosphere system (Goody and Yung, 1989; Kiehl and Trenberth, 1997; Roberto et al., 1999). The irradiation amount at the earth's surface is not uniform, and the annual value at the equator is 2.4 times that near the poles. The solar energy incident upon a surface depends on the geographic location, orientation of the surface, time of the day, time of the year, and atmospheric conditions (Boes, 1981).

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