Solar Radiation

Introduction

The sun emits electromagnetic radiation, with only a portion of the electromagnetic spectrum visible to the human eye. The region of the electromagnetic spectrum that contains light at frequencies and wavelengths that stimulate the rod and cones in the human eye is termed the visible region of the electromagnetic spectrum. Higher energy solar radiation (at energy levels and frequencies above light in the visible spectrum) includes x-rays, extreme ultraviolet (EUV) and ultraviolet (UV) radiation. Lower-energy electromagnetic radiation, including infrared light, microwaves, and radio waves, lies at lower frequencies and longer wavelengths than visible light.

Historical Background and Scientific Foundations

The high-energy portion of solar radiation that is directed toward Earth is absorbed by upper regions of the atmosphere. In the upper reaches of Earth's atmosphere, there is a region identified as the thermosphere where neutral atoms and molecules absorb EUV and UV light to heat the region to temperatures in the range of

1800°F (1000°C). The atoms and molecules become ionized at such temperatures. The upper layer of Earth's atmosphere, including the thermosphere region, is generally described as the ionosphere.

Most of the sun's harmful radiation is absorbed in the ionosphere. A very small amount of UV radiation, all of the visible spectrum and the infrared radiation, reach the lower atmosphere and the surface of Earth. About half of the solar radiation that reaches Earth in daylight hours is absorbed. The remainder is reflected back into space.

WORDS TO KNOW

ELECTROMAGNETIC SPECTRUM: The entire range of radiant energies or wave frequencies from the longest to the shortest wavelengths—the categorization of solar radiation. Satellite sensors collect this energy, but what the detectors capture is only a small portion of the entire electromagnetic spectrum. The spectrum usually is divided into seven sections: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray radiation.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

IONIZE: To add or remove electrons from an electrical neutral atom i.e., one surrounded by as many electrons as it has protons in its nucleus, therefore electrically neutral). An ionized atom is termed an ion and has a positive or negative electrical charge. Molecules (clusters of atoms bound stably together) may also become ionized.

IONOSPHERE: A subregion within the thermosphere, extending from about 50 mi (80 km) to more than 250 mi (400 km) above Earth and containing elevated concentrations of charged atoms and molecules (ions).

THERMOSPHERE: The outermost shell of the atmosphere, between the mesosphere and outer space; where temperatures increase steadily with altitude.

ULTRAVIOLET RADIATION: The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5% of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.

WAVELENGTH: Distance between the peaks or troughs of a cyclic wave. The character and effects of electromagnetic radiation are determined by its wavelength: very short-wavelength rays (e.g., X rays) are biologically harmful, somewhat longer-wavelength rays are classified as ultraviolet light, rays of intermediate wavelength are visible light, and longer wavelengths are infrared radiation and radio waves.

IN CONTEXT: SOLAR RADIATION

Despite uncertainty over the impacts of variation in solar radiation, based upon well-established data, Intergovernmental Panel on Climate Change (IPCC) scientists conclude: “From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols and land surface changes, it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750.” The IPCC adds that “[s]olar irradiance contributions to global average radiative forcing are considerably smaller than the contribution of increases in greenhouse gases over the industrial period.”

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solar radiation absorbed by clouds and greenhouse gases adds warmth at the surface and drives large scale ocean-atmospheric circulation patterns.

Impacts and Issues

According to the U.S. National Aeronautics and Space Administration (NASA) researchers, total solar radiation varies and the increases observed during recent “quiet time” (cycles of normally lower solar activity) could be a key factor driving non-human-caused climate change. Human contributions to pollution and greenhouse gases, however, greatly magnify the impact of solar radiation variations (usually less that 1%) to produce the potential for highly significant climate change.

See Also Solar Energy; Solar Illumination.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Web Sites

“Brief Descriptions of the Solar Spectrum and Solar-Terrestrial Effects.” National Oceanic and Atmospheric Administration (NOAA), 2007. <http://www.sec.noaa.gov/spacewx/Solar_Spectrum.html> (accessed August 31, 2007).

“Solar Physics.” National Aeronautics and Space Administration (NASA), 2007. <http://solarscience.msfc.nasa.gov/SMM.shtml> (accessed August 31, 2007).