Solar Constant

The Sun as an average star is a typical main-sequence dwarf of spectral class G-2. Its radius is 6.960 x 10⁸ m. The mean distance between the Sun and the Earth is 1.496 x 10¹¹m and is known as the astronomical unit (AU). Solar radiation is the electromagnetic radiation emitted by the Sun. Almost all known physical and biological cycles in the Earth system are driven by the solar radiation reaching the Earth. Solar radiation is also the cause of climate change that is truly exterior to the Earth system.

The distribution of solar radiation as a function of the wavelength is called the solar spectrum, which consists of a continuous emission with some superimposed line structures. The Sun’s total radiation output is approximately equivalent to that of a blackbody at 5776 K. The solar radiation in the visible and infrared spectrum fits closely with the blackbody emission at this temperature. However, the ultraviolet (UV) region (< 0.4 mm)of solar radiation deviates greatly from the visible and infrared regions in terms of the equivalent blackbody temperature of the Sun. In the interval 0.1–0.4mm, the equivalent blackbody temperature of the sun is generally less than 5776 K with a minimum of about 4500 K at about 0.16 mm. The deviations seen in the solar spectrum are a result of emission from the non-isothermal solar atmosphere.

The solar constant is the amount of solar radiation received outside the Earth’s atmosphere on a surface normal to the incident radiation per unit time and per unit area at the Earth’s mean distance from the Sun. The solar constant is an important value for the studies of global energy balance and climate. Reliable measurements of solar constant can be made only from space and a more than 20-year record has been obtained based on overlapping satellite observations. The analysis of satellite data suggests a solar constant of 1366 W m^-2 with a measurement uncertainty of 73Wm^-2.Of the radiant energy emitted from the Sun, approximately 50% lies in the infrared region, about 40% in the visible region, and about 10% in the UV region.

The solar constant is not in fact perfectly constant, but varies in relation to the solar activities. Beyond the very slow evolution of the Sun, a well-known solar activity is the sunspots, which are relatively dark regions on the surface of the Sun. The periodic change in the number of sunspots is referred to as the sunspot cycle, and takes about 11 years, the so-called 11-year cycle. The cycle of sunspot maxima having the same magnetic polarity is referred to as the 22-year cycle. The Sun also rotates on its axis once in about 27 days. Satellite observations suggest that the solar cycle variation of the solar constant is on the order of about 0.1%, which might be too small to directly cause more than barely detectable changes in the tropospheric climate. However, some indirect evidence indicates that the changes in solar constant related to sunspot activity may have been significantly larger over the last several centuries. Furthermore, solar variability is much larger (in relative terms)in the UV region, and induces considerable changes in the chemical composition, temperature, and circulation of the stratosphere, as well as in the higher reaches of the upper atmosphere.

Outside the earth’s atmosphere, solar radiation has an intensity of approximately 1370 watts/meter2. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant.