WSPR anomalies return.

With fairly quiet geomagnetic conditions returning over the past day, I decided to run the RSP1a receiver overnight, to see what surprises might come in at 14MHz.

Sure enough, for just a few minutes around 02:20UT, KK6L's 5W was heard - quite weakly - where he was received by no other station in the dark hemisphere, other than TF4AH, who, being under the auroral oval, has markedly different propagation conditions than I do.  You might miss my reception on the plot - it's the short blue line (just three spots), right of lower centre.

I was interested to see that ZL stations were also hearing KK6L, and so I plotted ZL1ROT alongside myself and TF4AH, to see what that might reveal in relation to the opposite hemisphere.

14MHz reception of KK6L, 2020 September 07

As you can see - and remarkably clearly so - the signal response in the southern hemisphere is in a very good inverse correlation with TF4AH; when his signal is reducing to then hover around the -17dB level, ZL1ROT's received signal strength increases to much the same level.  

If this isn't immediately clear, then look at TF4AH's response, which is essentially a 'U' shaped curve, and then ZL1ROT's response, which is an upside-down U shape.  A 3-degree polynomial trend curve may make things a little clearer:

The sudden appearance of KK6L comes as the field restored from a southern deviation to a solar-quiet condition, although on this occasion, the field had been quiet for about an hour before KK6L was heard.

The reception also came, as I have seen many times before, as the auroral oval's midnight edge came to lie mid-way between KK6L and the UK.  At this time also, KK6L is just 10 minutes before sunset, so his D layer is rapidly starting to diminish. 

I expect the signal strength increase with KK6L that happens around this time briefly reaches detectable levels here in Wales due to the assistance of very localised ionisation, apparently within the inner edge of the auroral oval, if the path is assumed (probably simplistically) to be a great circle:

Terminator and auroral oval at time of KK6L reception by MW1CFN

 

Great circle path KK6L to MW1CFN at 02:20UT, assuming no skew.