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Definition:
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The total radiant energy received vertically from the Sun, per unit area per unit of time, at a position just outside the Earth’s atmosphere when the Earth is at its average distance from the Sun. Radiation at all wavelengths from all parts of the solar disk is included. Its value is approximately 2.00 cal cm-2 min-1 = 1.37 kW m-2 and varies slightly (by approximately 0.1 %) from day to day primarily in response to the apparent size of sunspots blocking emission from the photosphere.
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