The Solar variation reference article from the English Wikipedia on 24-Jul-2004
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Solar variation

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Solar variation refers to fluctuation in the amount of energy emitted by the Sun. The Sun is a star undergoing hydrogen to helium fusion reactions due to compression deep inside. The rate of reactions is sufficiently constant that the amount of solar radiation emitted at the surface does not change much. Small variations have been measured from satellites during recent decades.

Table of contents
1 Sunspots
2 Solar cycles
3 Solar luminosity
4 Solar irradiance of Earth and its surface
5 Other effects due to solar variation
6 Global warming
7 Links


Sunspots are relatively dark areas on the surface of the Sun. They have been determined to be cooler than its average surface. Variations in their numbers alter the brightness of parts of the Sun.

Solar cycles

Solar cycles are cyclic changes in behavior of the Sun. Most obvious is a gradual increase and decrease of the number of sunspots over a period of about 11 years, called the Schwabe cycle. This seems to be due to a shedding of entangled magnetic fields. The Sun's surface is also the most active when there are more sunspots, although the luminosity does not change much due to an increase in bright spots (faculae). Other patterns detected are the Hale cycle (22 years) and the Gleissberg cycle (70-100 years).

Solar luminosity

The overall brighness of the Sun has been found to vary slightly during a few recent solar cycles. It has been most directly measured by satellites above the atmosphere. Comparisons between ground instruments, high-altitude instruments, and instruments in orbit have been used to calibrate ground instruments. Researchers have combined present readings and factors to adjust historical data. Also used have been proxy data, such as measurements of cosmic ray isotopes to infer solar magnetic activity and thus the likely brightness.

Solar irradiance of Earth and its surface

Solar irradiance is the amount of sunlight which reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies. Historical estimates use various measurements and proxies.

There are two common meanings:

Climate models are computer simulations which are used to examine understanding of climate behavior. Some models use constant values for solar irradiance, while some include the heating effects of a variable Sun.

Other effects due to solar variation

Interaction of solar particles, the solar magnetic field, and the Earth's magnetic field, cause variations in the particle and electromagnetic fields at the surface of the planet. Extreme solar events can affect electrical devices. Weakening of the Sun's magnetic field is believed to increase the number of interstellar cosmic rays which reach Earth's atmosphere, altering the types of particles reaching the surface. It has been speculated that a change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's albedo.

Global warming

Some researchers have correlated solar variation with changes in the Earth's average temperature and climate - sometimes finding an effect, and sometimes not. When effects are found they have tended to be greater than can be explained by direct response to the change in radiative forcing from solar change, so some kind of feedback or amplification mechanism is required. See anthropogenic climate change for a discussion of attribution of causes of current global warming.