I had written back in July 2008 about the 10.7cm solar radio flux hitting a new record low value. Part of that has to do with the inverse square law and the distance of the earth to the sun, which is at a maximum at the summer solstice. As you can see below there has been a very gradual rise since then as we approached the winter solstice. David Archibald provides an update below and compares our current period to other solar cycles. – Anthony
UPDATE: In comments, Leif Svalgaard offers his graph, and also speaks of the flatlining. See below the “read more” – Anthony
The graph above is of two year windows of the F 10.7 radio flux centered on the last five solar minima. They are stacked up so that they are 20 solar flux units apart on the same vertical scale. The original data is from: http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarradio.html#qbsa
That site notes:
“The quiet sun level is the flux density which would be observed in the absence of activity. Extrapolation to zero of plots of the 10.7cm flux against other activity indices such as plage area or total photospheric magnetic flux in active regions suggest a quiet sun flux density of about 64 s.f.u. This is rarely attained.” The lowest daily value in this minimum to date was 64.5 in June 2008.
What is evident is that this minimum is quite different from the previous four in that the intra-monthly amplitude has died from June 2008. The monthly average low was July 2008 and the series has been in uptrend at 0.7 units/month thereafter. This is a very weak but very consistent uptrend, perhaps the first sign of a rising Solar Cycle 24. There is very little noise in this signal, suggesting a very weak Solar Cycle 24.
– David Archibald
UPDATE: Leif Svalgaard writes in comments:
As part of my ‘homework’ for the Sunspot Panel [2 years ago] I produced a short document
comparing F10.7 and MgII [another solar index] around minima. I have updated the graph in the document to show the flat-lining of F10.7.