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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]
Total solar irradiance (TSI) [22] changes slowly on decadal and longer timescales. The variation during solar cycle 21 was about 0.1% (peak-to-peak). [23] In contrast to older reconstructions, [24] most recent TSI reconstructions point to an increase of only about 0.05% to 0.1% between the 17th century Maunder Minimum and the present.
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.
The relative spectral flux density is also useful if we wish to compare a source's flux density at one wavelength with the same source's flux density at another wavelength; for example, if we wish to demonstrate how the Sun's spectrum peaks in the visible part of the EM spectrum, a graph of the Sun's relative spectral flux density will suffice.
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.
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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In active regions the energy flux is about 10 7 erg cm −2 sec −1, in the quiet Sun it is roughly 8 10 5 – 10 6 erg cm −2 sec −1, and in coronal holes 5 10 5 - 8 10 5 erg cm −2 sec −1, including the losses due to the solar wind. [1] The required power is a small fraction of the total flux irradiated from the Sun, but this energy is ...