Solar Illumination

Introduction

Solar illumination is an integral part of total energy balance fundamental to both short-term weather events and long-range estimations of climate change. Atmospheric circulation is the cause of weather and is driven by energy received from the sun.

Solar illumination comes from the thermonuclear reactions within the sun that provide the energy emitted into space. Surface sunspots, solar flares, and coronal mass ejections are sources of variations in solar illumination. Earth's ionosphere protects it from much of the sun's emissions. Exactly how sensitive Earth's climate is to these variations is not well understood.

Historical Background and Scientific Foundations

Since Italian physicist Galileo (1564–1642) published a description of sunspots in 1613, scientists have observed the sun's surface changes. In 1845, Samuel Heinrich Schwabe (1789–1875), a German amateur astronomer, published an article on his 17-year study of sunspots that indicated there are cycles in sunspot frequency. Schwabe's studies launched modern sunspot research. Sunspot occurrences go from a minimum number of spots to a maximum number of spots in approximately 11 years.

Direct measurements of the amount of illumination reaching Earth's outer limits were not possible until satellite measurements in the later half of the 20th century. In 1973, coronal mass ejections were first observed from space. Since that time, measurements indicate that coronal mass ejections of giant plasma clouds away from the sun produce an abrupt energy release.

In addition, solar flares often erupt near sunspots emitting very high temperature material into space. Solar flares give off high energy ultraviolet radiation and x-rays that cause Earth's upper atmosphere to warm. In February 1980, the U.S. National Aeronautics and Space Administration (NASA) launched the Solar Max satellite to study solar flares. The satellite sampled solar radiation until 1989 when the increasing radiation due to approaching sunspot maximum caused Earth's orbit to expand and capture the satellite, slowing it by friction until it reentered and burned in Earth's atmosphere.

Despite intense research, questions remain about exactly what triggers sunspots, or their cyclic maximums and minimums. Current question include the structure and process that take place in the sun's outer atmosphere, the corona. The sun's corona is hotter than 1,800,000°F (1,000,000°C), actually hotter than just below the surface of the sun. The high temperatures of the corona cause solar wind, a flow of gases that travel at 1 million mph (500 km/s) past Earth. Solar winds increase the energy levels of Earth's ionosphere.

Impacts and Issues

The solar constant defines the amount of energy that reaches the outer limits of Earth's atmosphere. Scientists on the Intergovernmental Panel on Climate Change (IPCC) assert that the variation in the solar constant is negligible at 0.1%, but not all scientists agree with that assertion. Critics contend that a higher variation means that variables in solar illumination may carry far greater impacts on Earth's climate than currently estimated.

Solar illumination has varied over timescales that range from milliseconds to billions of years. Although total solar irradiation has changed only around 0.1% in recent decades, there are much longer cycles of millions of years that appear to be associated with greater variations leading to ice ages.

WORDS TO KNOW

CORONAL MASS EJECTIONS: Events in which masses of hot gas (plasma) are ejected from the sun's outer atmosphere, its corona. After several days, if a coronal mass ejection is headed the right way, it may intercept Earth, causing auroras and in some cases damaging electronic equipment and power transmission grids.

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).

PLASMA: Matter in the form of electrically charged atomic particles that form when a gas becomes so hot that electrons break away from the atoms. Also, the colorless, liquid portion of the blood in which blood cells and other substances are suspended.

THERMONUCLEAR REACTION: A nuclear reaction that takes place only at very high temperatures, usually on the order of a few million degrees.

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.

In order to study solar radiation, agencies such as NASA and the National Oceanic and Atmospheric Administration (NOAA) are continuously monitoring solar activities from space. Such studies will provide data that will help clarify what part variables in solar illumination, especially relative to the changes produced by human activities, drive climate change.

See Also IPCC Climate Change 2007 Report; Solar Energy; Solar Radiation.

BIBLIOGRAPHY

Web Sites

“Climate.” Cycles Research Institute, 2007. <http://www.cyclesresearchinstitute.org> (accessed August 29, 2007).

“Solar Physics.” National Aeronautics and Space Administration (NASA), 2007. <http:// solarscience.msfc.nasa.gov/SMM.shtml> (accessed August 29, 2007).

“The Sun-Climate Connection.” National Oceanic and Atmospheric Administration, 2007. <http://www.research.noaa.gov/spotlite/archive/spot_sunclimate.html> (accessed August 29, 2007).