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The solar constant includes radiation over the entire electromagnetic spectrum. It is measured by satellite as being 1.361 kilo watts per square meter (kW/m 2) at solar minimum (the time in the 11-year solar cycle when the number of sunspots is minimal) and approximately 0.1% greater (roughly 1.362 kW/m 2) at solar maximum. [1]
It is measured facing (pointing at / parallel to) the incoming sunlight (i.e. the flux through a surface perpendicular to the incoming sunlight; other angles would not be TSI and be reduced by the dot product). [3] The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU).
The solar flux unit (sfu) is a convenient measure of spectral flux density often used in solar radio observations, such as the F10.7 solar activity index: [1]. 1 sfu = 10 4 Jy = 10 −22 W⋅m −2 ⋅Hz −1 = 10 −19 erg⋅s −1 ⋅cm −2 ⋅Hz −1.
Then, in the region, I (x, t ; r 1, ν) takes a constant scalar value, which we here denote by I. In this case, the value of the vector flux density at P 1 is the zero vector, while the scalar or hemispheric flux density at P 1 in every direction in both senses takes the constant scalar value πI.
W⋅sr −1 ⋅m −3: M⋅L −1 ⋅T −3: Irradiance Flux density: E e [nb 2] watt per square metre W/m 2: M⋅T −3: Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity". Spectral irradiance Spectral flux density: E e,ν [nb 3] watt per square metre per hertz W⋅m −2 ⋅Hz −1: M⋅T −2
The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7 cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or solar flux units (1 SFU = 10 −22 W m −2 Hz −1). It represents a measure of diffuse, nonradiative coronal plasma heating.
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Solar radiation pressure on objects near the Earth may be calculated using the Sun's irradiance at 1 AU, known as the solar constant, or G SC, whose value is set at 1361 W/m 2 as of 2011. [17] All stars have a spectral energy distribution that depends on their surface temperature. The distribution is approximately that of black-body radiation.