international solar flux unit

A unit used by astronomers to express the flux density of radio energy from the sun as received at the Earth. One solar flux unit = 10⁻²² watt per square meter-hertz. Symbol, sfu. 1 sfu = 10,000 jansky.

The solar flux density is proportional to the sunspot number. Typically, the solar flux density is measured at a wavelength of 10.7 centimeters (approximately corresponding to a frequency of 2800 megahertz), because these measurements correlate with the sun's output in the ultraviolet. Values range from 67 sfu (when the sun shows no sunspots) to 300 sfu, but bursts can be much higher. The record is 55,000 sfu, recorded on 6 June 1991.

Radio amateurs who delight in receiving distant stations (DX'ers) take great interest in sfu, because the long distance transmission of shortwave signals depends on reflecting signals off the ionosphere, and the state of the ionosphere depends on the solar flux. At 18 minutes past the hour, the NIST radio station WWV (Fort Collins, CO) broadcasts a measurement in sfu of the solar radio flux at 10.7 cm (2800 MHz), obtained from a radio telescope at the Penticton Radio Observatory in British Columbia. Readings are updated every three hours, beginning at midnight UTC. The text of the current report can be accessed through the web at


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