Solar cycle#Solar radio flux

The solar radio flow index (English Radio solar flux, reduces solar flux, and F10.7 Index ) is a measure of the solar radiation at the radio frequency of 2800 MHz corresponding to a wavelength of 10.7 cm. The radiation intensity is also referred to as 10.7 cm radiation, radio flow index F10.7, Covington Index ( CI) or solar flux.

Importance

The radio flux index is proportional to the solar activity. It correlates with the number of sunspots, but is easier to measure than the sunspot number. Was discovered in this context by Arthur Covington, who could confirm his suspicions in 1946 during a solar eclipse.

The radiation density is measured in watts per square meter per hertz of bandwidth, sometimes expressed in Jansky, but mostly in solar flux units ( sfu ):

The solar flux is measured daily from stations on the ground. It is suitable for measuring solar activity better than the sunspot number, as it depends on the subjective, manual counting of sunspots.

Correlation with the sunspot number

The relative sunspot number R correlates very strongly with the radio wave radiation from the Sun at 10.7 cm wavelength.

The conversion factors of R in sfu denominated in F:

The values ​​refer to the mean Earth-Sun distance. Thus, the absolute value of F = almost 10% fail 67 sfu at R = 0 in the summer smaller when the earth is farther away on their elliptical orbit of the sun. Typically, F is 70 sfu at low solar activity and reaches solar maximum partially over 200 units.

In the literature one finds the following relation between R and F ( in sfu ):

Use

The solar radio flux index is inter alia to predict the propagation of short-wave radio used because the ionization of the ionosphere, which is necessary to earth the back reflection of the short waves is directly related to solar activity. However, the solar radiation of that frequency is not itself able to ionize the ionosphere. This is mainly due to solar ultraviolet radiation, in particular X-ray. For this reason, the measurement value of the satellites determined by the X-ray flux is used as a basis for the ionization.

Another reason for its use for prediction purposes is the property of the solar flux to wander around by refraction at the solar limb. It is used to predict so-called active longitudes - active solar regions - which are still on the back of the sun, but soon appear on the eastern edge.