Something of a radiation mystery...?

I recently started looking at the output of some of the other radiation stations on this site and comparing against my own (station: cowpip). One station in particular really has me puzzled. The station is davidnicholson's station in Airdrie Alberta. As the crow flies, our stations are about 228 km (141 miles) apart. David's station has a particularly clean (almost anomalously so) signal that shows the diurnal pattern particularly well. When I compare it against my own readings using a PRM-9000 Geiger counter, a VERY similar diurnal pattern emerges (except my signal is much dirtier with random noise). Aside from a difference in latitude of only about 1 degree of latitude, our altitudes are almost identical. I haven't yet studied the data in detail, but it looks like the time coincidence of the paterns between our monitors is remarkably similar.

I have found some higher latitude European stations (like Natrix in Sweden) whose peaks and valleys are not that dissimilar from what we are seeing on a diurnal basis (look at the weekly charts). And I read a blog post from fourmilab.com (in Switzerland) about an interesting diurnal pattern that was observed there some years ago, which aligns nicely with what I am seeing. The apparent diurnal peak in Sweden very closely matches the apparent diurnal peak seen at Fourmilab (occuring near 7 am local time).

What is interesting to me, and somewhat inexplicable - if any of this is real - is that the peaks and valleys my station and davidnicholson are seeing, seem to be very similarly time correlated with what Europe sees. In other words, we see a diurnal increase near the same time that they see a diurnal increase. Even though they are about 8 hours ahead of us, our monitors seem to respond at similar times to this increase in radiation. I can't find a similar pattern in the southern hemisphere, or at lower latitudes (although a Denver station I have checked looks interesting - but Denver is also higher in altitude than my station - by like 500 to 700 meters).

I am curious if anyone here might have ideas as to what could produce these seemingly coincident signals. I can explain the lack of a signal from lower latitudes due to possible effects of magnetic cutoff-rigidity. Lower latitudes would need higher energy cosmic rays to register at ground-level. I have also considered functions of geomagnetic activity, but that wouldn't explain how stations on near-opposite sides of the world would see signatures that kind of resemble each other. Sweden has a similar geomagnetic latitude to my location, even though it is higher in geographic latitude, so any cosmic ray response should probably not be that different if the cosmic ray signatures are like waves that sweep the entire Earth? It would not account for localized cosmic rays that should be very localized indeed.

I can't think of anything that could cause this anomaly. It commences around 1 am local time for me and around 8 or 7 am local time in Sweden or Switzerland. Although this is a significant stretch, if the source were astronomical and located pole-ward in the sky, it might help explain why there is an asymmetry in the northern and southern hemispheres (the southern hemisphere couldn't see the source, but the entire northern hemisphere could - and entry into the northern polar ground regions would be easier through the magnetic poles thanks to lower rigidity. But the diurnal pattern suggests a local source - or at least a local modulating source.

But this is all just speculation right now. I don't recall seeing this signature when I first fired up my monitor more than a year ago. And since I can now correlate it with nearby station measurements of a completely different design, I have to wonder what is causing it?

I have not looked at the data you mentioned, but one thing I recently learned about is how ground-level Radon seeping from the earth is affected by ground water levels, so a short dry period with no water in the upper soil from rain (which blocks gas movement), and also lowered water table levels from a longer dry spell both affect the 'reservoir' of Radon that can be easily released up to the surface. Air pressure drop, caused mostly by diurnal temperature changes brings Radon up, and if there is no wind to disperse it and/or if there is a temperature inversion trapping air at the surface, we will get higher readings, particularly from outdoor detectors. In my occasional sampling of station data with daily spikes, I have seen most often spikes in the morning, and less often at other times. Maybe the spikes at 1 am and at 7 am at the 2 locations are due to local temperatures/pressures that happen to coincide? I got most of my info from this report:
"Generic EIS in Support of Rulemaking on Radiological Criteria for Decommissioning of NRC-licensed Nuclear Facilities, 1994"
https://play.google.com/store/books/details?id=GfI3AQAAMAAJ&rdid=book-GfI3AQAAMAAJ&rdot=1
(Note browser cookies are required to view the book for free). Pages A-10 to A-12 describe a typical 11 day graph with daily changes.
From the station graphs I saw, the daily patterns don't seem to last long; mostly several days at a time which matches typical weather pattern duration. My guess is that the similarity with Europe is a coincidence and will soon change.

I have not found any studies that use local weather data to predict Radon levels; maybe there are too many variables or maybe no-one has seriously tried it. It would be very useful to "subtract" such predicted spikes from our data to avoid unnecessary alerts.

As what I assume is happening here at my homestead as well and we are in a severe drought. Water table so low I'm getting rust from my 4 inch., 55 foot deep cast iron pipe in the ground for seveal weeks now and my indoor radiation count has doubled, at times, not all the time, but has its moments. Main river in front of my house, 200 yards away is down 6 to 8 feet below the norm here.