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The Solar Spectrum

The Solar Constant

The Solar Constant, S, is the integrated solar spectral irradiance over all wavelengths and reported in Systeme International (SI) SI units of W·m-2 corrected to 1 Astronomical Unit (AU). An AU is the mean Sun-Earth distance. The accepted value of S has changed over time and it currently has a community-accepted value of 1366.1 W·m-2. This is the derived mean of daily averages from six different satellites over the 1978-1998 time period which all measured the total solar irradiance with absolute cavity radiometers and has been reported by Fröhlich and Lean. The standard deviation of this mean value is 425 ppm, with a 0.37 % minimum-maximum range (1363-1368 W·m-2).

solar spectrum 144 pixels

The SOLAR2000 solar spectrum from 1 to 1,000,000 nm


Solar Irradiance Spectral Categories

Definitions are based on the recommendations or usage by provider/user communities. There have been differing definitions used by several communities and this description does not recommend one definition over another in cases where differences exist. These definitions may be changed in the future as convention dictates.

The units used are a combination of SI units in nanometers (1 nm = 1E-9 meters) and community recognized units. These include microns (1 µ = 1E-6 meters), millimetres (1 mm = 1E-3 meters), centimetres (1 cm = 1E-2 meters), and Hertz (cycles s-1) (Hz = c/lambda where c is the speed of light 299792458 m s-1 and lambda is the wavelength of interest in meters; for example, the 10.7 cm radio flux can be converted to Hz with the following calculation: (299792458)/(0.107 m) = 2801.799 MHz; kHz = 1E3 Hz; MHz = 1E6 Hz; GHz = 1E9 Hz).

        1. Hard X-rays. Hard X-ray irradiances are defined in the wavelength range of 1 nm > lambda. This wavelength range is commonly used by providers of the data of this spectral range.
        2. Soft X-ray. irradiances are defined in the wavelength range of 10 nm > lambda ≥ 1 nm. This wavelength range is commonly used by providers of the data of this spectral range. The aeronomy community sometimes considers soft X-rays, also called the XUV, to extend to 30 nm.
        3. Utraviolet. Ultraviolet wavelengths have been assigned a variety of divisions, depending upon user community, measurement techniques, and instrumentation capabilities.
          • Ultraviolet. Ultraviolet or UV irradiances are defined in the wavelength range of 400 nm > lambda ≥ 100 nm as part of the Global Solar UV Index (UVI) designation.
          • Vacuum Ultraviolet. Vacuum Ultraviolet or VUV irradiances are defined in the wavelength range of 200 nm > lambda ≥ 10 nm. This wavelength range is commonly used by providers of the data of this spectral range as well as by the materials sciences community.
          • Extreme Ultraviolet. Extreme ultraviolet or EUV irradiances are defined in the wavelength range of 100 nm > lambda ≥ 10 nm. This wavelength range is commonly used by providers of the data of this spectral range. Some members of the aeronomy community have used a definition with the lower wavelength cut-off at 30 nm and a higher cut-off at 120 nm.
          • Far Ultraviolet. Far Ultraviolet or FUV irradiances are defined in the wavelength range of 200 nm > lambda ≥ 100 nm. This wavelength range is commonly used by providers of the data of this spectral range. Some members of the aeronomy community consider the start of this wavelength range at 120 nm.
          • Ultraviolet C. Ultraviolet C or UVC irradiances are defined in the wavelength range of 280 nm > lambda ≥ 100 nm as part of the Global Solar UV Index (UVI) designation.
          • Middle Ultraviolet. Middle Ultraviolet or MUV irradiances are defined in the wavelength range of 300 nm > lambda ≥ 200 nm. This wavelength range is commonly used by the aeronomy community.
          • Ultraviolet B. Ultraviolet B or UVB irradiances are defined in the wavelength range of 315 nm > lambda ≥ 280 nm as part of the Global Solar UV Index (UVI) designation.
          • Near Ultraviolet. Near ultraviolet or NUV irradiances are defined in the wavelength range of 400 nm > lambda ≥ 300 nm. This wavelength range is commonly used by the aeronomy community.
          • Ultraviolet A. Ultraviolet A or UVA irradiances are defined in the wavelength range of 400 nm > lambda ≥ 315 nm as part of the Global Solar UV Index (UVI) designation.
        4. Visible. Visible or optical or VIS irradiances are defined in the wavelength range of 700 nm > lambda ≥ 400 nm.
        5. Infrared. Infrared or IR irradiances are defined in the wavelength range of 350 000 nm (350µm) > lambda ≥ 700 nm (0.70 µm). Infrared is often divided into 3 spectral categories, i.e., the near, mid and far-infrared. The definitions are not formally agreed upon.
          • Near Infrared. Near infrared irradiances are often defined in the wavelength range of 5 000 nm (5 µm) > lambda ≥ 700 nm (0.70 µm).
          • Mid Infrared. Mid infrared irradiances are often defined in the wavelength range up to 40 000 nm (40 µm) > lambda ≥ 5 000 nm (5 µm).
          • Far infrared. Far infrared irradiances are often defined in the wavelength range up to 350 000 nm (350 µm) > lambda ≥ 40 000 nm (40 µm).
        6. Microwave. Microwave irradiances are often defined in the wavelength range of 15 000 000 nm (1.5 cm) > lambda ≥ 1 000 000 nm (1 mm). Defined microwave bands include the W (lambda = 3 300 000 nm or 3.3 mm), V (lambda = 5 000 000 nm or 5.0 mm), Q (lambda = 7 500 000 nm or 7.5 mm), Ka (lambda = 10 000 000 nm or 10.0 mm), and K (lambda = 13 600 000 nm or 13.6 mm).
        7. Radio. Solar radio irradiances are often defined in the wavelength range of 100 000 000 000 nm (100 m or ~3000 kHz) > lambda ≥ 100 000 nm (0.1 mm or ~3000 GHz) although most reports of solar measurements range from 10 000 000 000 nm (10 m or ~30 MHz) > lambda ≥ 1 000 000 nm (1.0 mm or ~300 GHz).
        8. Total Solar Irradiance (TSI) is the full-disk (whole Sun) solar irradiance at 1 AU integrated across all wavelengths.