Total solar irradiance (TSI), the so-called Solar Constant, is the total amount of electromagnetic energy irradiated from the Sun over the entire spectrum observed at the top of the Earth's atmosphere per unit area and per unit time. The spectrally distributed total solar irradiance is the Spectral Solar Irradiance (SSI). Solar irradiance is the driving term in the Earth's energy budget, and its variations are reckoned to be the prime solar factors exerting influence on the Earth's long-term climate change. The absorbtion, scattering, reflection of the solar energy by the Earth's atmosphere, clouds, or surface depend greatly on wavelength. Irradiance of different wavelength comes from different regions of the sun's atmosphere, thus the variations of SSI give us much infomation about solar activities at different heights of the solar atmosphere. The study of the variations of solar irradiance has important implications for our understanding of solar internal structure, solar activities, solar-terrestrial relationship, earth's atmosphere, as well as the global changes in the Earth's climate system. In Sect. 2 the measurements of both total and spectral solar irradiance are reviewed, and problems which still exist in the three composite data are discussed. Sect. 3 describes the reconstruction of solar irradiance by three different methods, including reconstruction from proxy indices, from intensity images, and from the measurements of surface magnetic field. The differences between different methods are also indicated. The progress of long-term reconstruction of solar irradiance and its application to climate research is reviewed. Utilizing the powerful methods, wavelet analysis and empirical mode decomposition, we studyed the PMOD composite of daily TSI, and the results are described in Sect. 4.
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