Insolation and total solar irradiance

Total solar irradiance is defined as the amount of radiant energy emitted by the Sun over all wavelengths that fall each second on 11 ft² (1 m²) outside Earth's atmosphere. Insolation is the amount of solar energy that strikes a given area over a specific time, and varies with latitude or the seasons .

By way of further definition, irradiance is defined as the amount of electromagnetic energy incident on a surface per unit time per unit area. Solar refers to electromagnetic radiation in the spectral range of approximately 1–9 ft (0.3–3 m), where the shortest wavelengths are in the ultraviolet region of the spectrum, the intermediate wavelengths in the visible region, and the longer wavelengths are in the near infrared. Total solar irradiance means that the solar flux has been integrated over all wavelengths to include the contributions from ultraviolet, visible, and infrared radiation.

By convention, the surface features of the Sun are classified into three regions: the photosphere, the chromosphere, and the corona. The photosphere corresponds to the bright region normally visible to the naked eye. About 3,100 mi (5,000 km) above the photosphere lies the chromosphere, from which short-lived, needle-like projections may extend upward for several thousands of kilometers. The corona is the outermost layer of the Sun; this region extends into the region of the planets. Most of the surface features of the Sun lie within the photosphere, though a few extend into the chromosphere or even the corona.

The average amount of energy from the Sun per unit area that reaches the upper regions of Earth's atmosphere is known as the solar constant; its value is approximately 1,367 watts per square meter. As Earth-based measurements of this quantity are of doubtful accuracy due to variations in Earth's atmosphere, scientists have come to rely on satellites to make these measurements.

Although referred to as the solar constant, this quantity actually has been found to vary since careful measurements started being made in 1978. In 1980, a satellite-based measurement yielded the value of 1,368.2 watts per square meter. Over the next few years, the value was found to decrease by about 0.04% per year. Such variations have now been linked to several physical processes known to occur in the Sun's interior, as will be described below.

From Earth, it is only possible to observe the radiant energy emitted by the Sun in the direction of our planet; this quantity is referred to as the solar irradiance. This radiant solar energy is known to influence Earth's weather and climate , although the exact relationships between solar irradiance and long-term climatological changes, such as global warming , are not well understood.

The total radiant energy emitted from the Sun in all directions is a quantity known as solar luminosity. The luminosity of the Sun has been estimated to be 3.8478 × 10²⁶ watts. Some scientists believe that long-term variations in the solar luminosity may be a better correlate to environmental conditions on Earth than solar irradiance, including global warming. Variations in solar luminosity are also of interest to scientists who wish to gain a better understanding of stellar rotation , convection, and magnetism .

Because short-term variations of certain regions of the solar spectrum may not accurately reflect changes in the true luminosity of the Sun, measurements of total solar irradiance, which by definition take into account the solar flux contributions over all wavelengths, provide a better representation of the total luminosity of the Sun.

Short-term variations in solar irradiation vary significantly with the position of the observer, so such variations may not provide a very accurate picture of changes in the solar luminosity. But the total solar irradiance at any given position gives a better representation because it includes contributions over the spectrum of wavelengths represented in the solar radiation.

Variations in the solar irradiance are at a level that can be detected by ground-based astronomical measurements of light. Such variations have been found to be about 0.1% of the average solar irradiance. Starting in 1978, space-based instruments aboard the Nimbus 7 Solar Maximum Mission, and other satellites began making the sort of measurements (reproducible to within a few parts per million each year) that allowed scientists to acquire a better understanding of variations in the total solar irradiance.

Variations in solar irradiance have been attributed to the following solar phenomena: Oscillations, granulation, sunspots, faculae, and solar cycle.

Oscillations, which cause variations in the solar irradiance lasting about five minutes, arise from the action of resonant waves trapped in the Sun's interior. At any given time, there are tens of millions of frequencies represented by the resonant waves, but only certain oscillations contribute to variations in the solar constant.

Granulation, which produces solar irradiance variations lasting about 10 minutes, is closely related to the convective energy flow in the outer part of the Sun's interior. To the observer on Earth, the surface of the Sun appears to be made up of finely divided regions known as granules, each from 311—1,864 mi (500–3,000 km) across, separated by dark regions. Each of these granules makes its appearance for about 10 minutes and then disappears. Granulation apparently results from convection effects that appear to cease several hundred kilometers below the visible surface, but in fact extend out into the photosphere, i.e., the region of the Sun visible to the naked eye. These granules are believed to be the centers of rising convection cells.

Sunspots give rise to variations that may last for several days, and sometimes as long as 200 days. They actually correspond to regions of intense magnetic activity where the solar atmosphere is slightly cooler than the surroundings. Sunspots

appear as dark regions on the Sun's surface to observers on Earth. They are formed when the magnetic field lines just below the Sun's surface become twisted, and then poke though the solar photosphere. Solar irradiance measurements have also shown that the presence of large groups of sunspots on the Sun's surface produce dips ranging in amplitude from 0.1 to 0.25% of the solar constant. This reduction in the total solar irradiance has been attributed both to the presence of these sunspots and to the temporary storage of solar energy over times longer than the sunspot's lifetime. Another key observation has been that the largest decreases in total solar irradiance frequently coincide with the formation of newly formed active regions associated with large sunspots, or with rapidly evolving, complex sunspots. Sunspots are especially noteworthy for their 11-year activity cycle.

Faculae, producing variations that may last for tens of days, are bright regions in the photosphere where high-temperature interior regions of the Sun radiate energy. They tend to congregate in bright regions near sunspots, forming solar active regions. Faculae, which have sizes on the order of 620 mi (1,000 km) or less, appear to be tube-like regions defined by magnetic field lines. These regions are less dense than surrounding areas. Because radiation from hotter layers below the photosphere can leak through the walls of the faculae, an atmosphere is produced that appears hotter, and brighter, than others.

The solar cycle is responsible for variations in the solar irradiance that have a period of about 11 years. This 11-year activity cycle of sunspot frequency is actually half of a 22-year magnetic cycle, which arises from the reversal of the poles of the Sun's magnetic field. From one activity cycle to the next, the north magnetic pole becomes the south magnetic pole, and vice versa. Solar luminosity has been found to achieve a maximum value at the very time that sunspot activity is highest during the 11-year sunspot cycle. Scientists have confirmed the length of the solar cycle by examining tree rings for variations in deuterium-to-hydrogen ratios. This ratio is temperature-dependent because deuterium molecules, which are a heavy form of the hydrogen molecule, are less mobile than the lighter hydrogen molecules, and therefore less responsive to thermal motion induced by increases in the solar irradiance.

Surprisingly, the Sun's rotation, with a rotational period of about 27 days, does not give rise to significant variations in the total solar irradiance. This is because its effects are over-ridden by the contributions of sunspots and faculae.

Scientists have speculated that long-term solar irradiance variations might contribute to global warming over decades or hundreds of years. More recently, there has been speculation that changes in total solar irradiation have amplified the greenhouse effect , i.e., the retention of solar radiation and gradual warming of Earth's atmosphere. Some of these changes, particularly small shifts in the length of the activity cycle, seem to correlate rather closely with climatic conditions in pre- and post industrial times. Whether variations in solar irradiance can account for a substantial fraction of global warming over the past 150 years, however, remains a highly controversial point of discussion.

See also Electromagnetic spectrum; Greenhouse gases and greenhouse effect; Solar energy; Solar illumination: Seasonal and diurnal patterns