Composition and power
The spectrum of the Sun's solar radiation can be compared to that of a black body with a temperature of about 5,800 K (see graph). The Sun emits EM radiation across most of the electromagnetic spectrum. Although the radiation created in the solar core consists mostly of x rays, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the photosphere of the Sun does not emit much X radiation (solar X-rays), although it does emit such "hard radiations" as X-rays and even gamma rays during solar flares. The quiet (non-flaring) Sun, including its corona, emits a broad range of wavelengths: X-rays, ultraviolet, visible light, infrared, and radio waves. Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.
There is also a flux of gamma rays from the quiescent Sun, obeying a power law between 0.5 and 2.6 TeV. Some gamma rays are caused by cosmic rays interacting with the solar atmosphere, but this does not explain these findings.
The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting neutrinos.
Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all (roughly 98.7%) of the solar electromagnetic radiation striking the Earth's atmosphere spans a range of 200 nm to about 4000 nm. This band of significant radiation power can be divided into five regions in increasing order of wavelengths:
Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn. In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer, but is also required for vitamin D synthesis in the skin of mammals.
Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and can cause cancer.
Visible range or light spans 380 to 700 nm. As the name suggests, this range is visible to the naked eye.
Infrared range that spans 700 nm to 1,000,000 nm (1 mm). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
Infrared-A: 700 nm to 1,400 nm
Infrared-B: 1,400 nm to 3,000 nm
Infrared-C: 3,000 nm to 1 mm.
The sunlight reaching Earth's surface is 49.4% infrared, 42.3% visible, and 8% ultraviolet.
It is sometimes asserted that the Sun's maximum output is in the visible range. However, this statement is a misconception based on only seeing the solar spectral irradiance plotted on a per-wavelength basis. When plotted that way, the power spectral density of sunlight peaks at a wavelength of about 501 nm, which is in the visible range. However, the solar spectral irradiance can with equal validity be calculated on a per-frequency basis, in which case the maximum is at 3.40×1014 Hz, corresponding to a wavelength of about 882 nm, which is in the near infrared (Infrared-A) range. Counterintuitively, it is not meaningful to assert that the solar output is greatest at some precise location in the spectrum.
