On July 18 1996, the observed value of the 10 cm solar flux dropped to a low of 64.9. 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 IPS Solar Geophysical Summary show 67.0 as the minimum value. So how can we get a value of 64.9?

The answer is quite interesting - it depends on the orbit of the earth! 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 10cm 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 to the sun and the solar minimum conditions have acted to produce this very low flux value.

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 corrected for the earth-sun distance variation.

The last time that the observed 10cm flux was at a lower value was on July 26, 1964 when it stood at 64.8. The lowest value ever recored was on July 02, 1954 with a value of 64.4.

Material Prepared by Richard Thompson. © Copyright IPS - Radio and Space Services.