A Very Low Ten Centimetre Solar Radio Flux
At low periods of a solar cycle, the observed daily value of the 10 cm solar flux can drop to below 67. In many books it is stated that the 10 cm solar flux can not go below a value of 67. For example, the formulae given in the June 1996 edition of the SWS Solar Geophysical Summary show 67.0 as the minimum value. So how can we this be?
The answer is quite interesting - it depends on the orbit of the Earth and the solar cycle! The Earth's orbit is not perfectly circular but is slightly elliptical. In July of each year we are a little further than average from the Sun and so solar radiation, including the 10 cm flux, is very slightly weaker than average.
So the 10 cm flux will tend to be lower in July than, for example, December when the Earth is closer to the Sun than its average value. The combination of the extra distance from the Sun and solar minimum conditions act to produce very low flux values.
It is easy to correct for the Earth-Sun distance and, when this is done, a value of 67.0 is obtained. This is the text book value!
Values of the 10 cm flux are often given in two forms - first as directly observed values and secondly as values adjusted for the Earth-Sun distance variation.
The lowest values ever recorded occurred in 1953 and 1954. Low values were also recorded in 1985, 1996 and 2008 - all near solar minimum.
Material prepared by Richard Thompson