Spectrum of Solar Radiation

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Figure 9.1 shows the incident UV, visible, and near-infrared parts of the spectral solar irradiance (wavelengths shorter than 1,000 nm) measured on board an earth-orbiting satellite (Rottman et al., 1993). Spectra of an ideal blackbody at several temperatures are also shown in Fig. 9.1. Given the requirement that the total solar energy emitted be the same as that emitted by a blackbody, one finds that the sun's effective temperature is 5,778 K. If the radiating layers of the sun had the same temperature at all distances from its center, the solar spectrum would

Figure 9.1 shows the incident UV, visible, and near-infrared parts of the spectral solar irradiance (wavelengths shorter than 1,000 nm) measured on board an earth-orbiting satellite (Rottman et al., 1993). Spectra of an ideal blackbody at several temperatures are also shown in Fig. 9.1. Given the requirement that the total solar energy emitted be the same as that emitted by a blackbody, one finds that the sun's effective temperature is 5,778 K. If the radiating layers of the sun had the same temperature at all distances from its center, the solar spectrum would

Extraterrestrial Radiation

Wavelength (nin)

Figure 9.1 Extraterrestrial solar irradiance. The UV spectrum between 119 nm and 420 nm was measured by the SOLSTICE (SOLar STellar Irradiance Comparison Experiment) instrument on the Upper Atmosphere Research Satellite (UARS) (Rottman et al., 1993). The smooth curves are calculated blackbody spectra for several temperatures (adapted from Thomas and Stamnes, 1999)

match one of the theoretical blackbody curves exactly. Thus, the deviations between the measured solar irradiance in Fig. 9.1 and one of the blackbody curves are the result of emission from a non-isothermal solar atmosphere. Most of the solar emission arises within the photosphere where the sun's visible optical depth reaches unity. The fine structure in Fig. 9.1 is due to Fraunhofer absorption by gases in the cooler (more distant) portions of the photosphere. The effective radiating temperature falls to values as low as 4,500 K at wavelengths between 125 nm and 380 nm. Additionally, the UV irradiance is noticeably dependent upon the solar cycle, being more intense at high solar activity than at low solar activity. Thus, the solar illumination varies on diurnal, seasonal as well as the 11-year solar cycle time scales (Thomas and Stamnes, 1999). The 11-year solar cycle is associated with the variation in the number of sunspots.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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