Low SFI and High MUF?

[Drax Spacex]

The current short-term trend for 10.7cm SFI is lower, correlated with fewer sunspots.  Currently SFI=129, SN=39.

Usually this means reduced skywave radio propagation.

However, daytime Maximum Usable Frequency (MUF) has often been above 35MHz during this period (currently 35.44MHz).  MUF is commonly positively correlated to SFI and the degree of ionization of the ionosphere.

So it seems the ionosphere has been storing up this energy like a flywheel (in concept not mechanics) from previous weeks when SFI an SN were higher.  At the same time instantaneous D-layer absorption of radio waves is reduced with lower SFI.  This  may create a prime ionospheric configuration that allows radio waves with high MUF to punch through the D-Layer followed by coherent refraction of radio waves from well-ionized E and F layers.

Edited October 24, 2023 by Drax Spacex
MUF correlates to SFI

[hamateur 1953]

Well said Drax.  Yes our 10 meter (28 mhz ) Ham band should be wide open during daylight hours now.  There are also seasonal variations in the F layer height and density that have been studied before.  As you may know @KW2Pand me are going to attempt a coast to coast CW ( morse code) contact this winter sometime.  It would be nice if the 200 sfi we have been hoping for to do this on our six meter band ( 50 mhz) turned out to be closer to 180 sfi.  That’s  a good bit of luck. And timing.  Haha.  We will see.  Mike/Hagrid. 

[hamateur 1953]

I pulled this topic forward because I have been observing some really unusual things.   Muf hardly dropped at all during the recent plunge in SFI.  UV radiation in the longer wavelengths   Is the primary source of F layer ionisation in addition to its somewhat slower response time to recombination.  This seems to make sense to me anyway.  I would imagine, therefore that the longer wavelengths of F30 would be a better proxy for F layer propagation worldwide. But this is only an opinion. 73 Mike N7ORL