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Total Solar Irradiance
The radiation from the Sun is the prime energy source together with the reflected and the emitted radiation from the Earth that drives the climate of the Earth as well as the dynamics of its atmosphere. In the actual context about the "Global Climate change" of the Earth, it is well recognized that the knowledge of the short and long term behavior of the "Solar Constant" (solar irradiance at a distance from the Sun of 1 astronomical unit) is of paramount importance. It is known since only one decade that the solar constant changes with the solar cycle.
The problem is to know how this behavior is linked to the peculiarities of the solar cycles inclusive eventual long term changes, drifts or variations due for instance on solar effects like diameter changes or else. To obtain this knowledge, it is imperative to perform absolute measurements of the highest possible accuracy and precision (resolution) of the solar irradiance, as a long term observation program.
TSI absolute measurement since 1978
Daily mean TSI measurements from DIARAD/VIRGO on SOHO
The DIfferential Absolute RADiometer (DIARAD) is one of the instruments of VIRGO onboard SOHO, DIARAD is measuring the TSI since 1996 with three minutes sampling time. DIARAD/VIRGO is composed of two channels, DIARAD left and DIARAD right. DIARAD left channel is continiousely measuring while DIARAD right channel is used as backup channel to correct the aging effect. Below is the ageing corrected daily mean measurements from DIARAD.
The TSI composite from the Royal Meteorological Institute of Belgium (Dewitte & al. , Mekaoui & Dewitte ) is constructed using the Space Absolute Radiometric Reference (SARR) (Crommelynck 1995). Below is a plot of the composite since 1985.