Crossing hemispheres at 6m.

Last Sunday (11/07/2021), there was a long-haul DX opening on 6m, bringing multiple QSOs between Europe, Australia and New Zealand.  EI7GL, as always, has the best take on events here.

These events are by no means unprecedented.  But they remain interesting, not least in terms of the propagation mechanism involved.

Sporadic E would appear to be the most obvious candidate mechanism in conveying these signals across vast, inter-hemisphere distances.  But it poses the problem of how many Es 'clouds' ('hops') are needed, and how they might align so well that it occurs at all.  

One idea might lie in the timing.  It seems to me - admittedly as a non-6m specialist - that many of these events, if not all, occur close to midsummer.  From a station based in Britain's viewpoint, that means long-haul DX on 6m in June/July, and perhaps also in December (when it is midsummer in the southern hemisphere).

A notable event happened at 12 and 10m - I'm not sure if it affected 6m - on December 01, 2019.  This coincided with the first ever confirmed sighting of noctilucent clouds - which occur at much the same height and are forced by much the same mechanisms as Es - from New Zealand. 

Now, in 2014, using data from the AIM satellite, which has now been working without much fault for some 14 years (and there is no planned replacement), an entirely new 'teleconnection' was discovered.  Teleconnections are most simply understood as large-scale atmospheric flows between well-separated regions of the Earth's atmosphere - in this case, hemispheres.  Video below explains:

 

The particles around which NLC form - and which also are key to Es formation - are extremely small (nanometere scale), smoke-like remains of meteor burn-ups.  Every day, several tens to a few hundred tonnes of material enters Earth's atmosphere.  So there is plenty of it, and it is very easily transported by the atmosphere, and shaped by wind shear, amongst other mechanisms.

Indeed, as the noctilcuent cloud season moves past midsummer, into July, the appearance of displays tends to change into long streaks that appear to flow from north to south, against a general backdrop of E-W motion.  This is often an effect of perspective.  But it can also be considered, at least in order to help visualise what's going on - a visible signature of meteoric debris - coated in reflective ice and making up a highly charged surface - flowing between the poles.   

NLC display in Sweden (11/07/2014), showing what can appear to be N-S flows of air at 82km up.  Image: Gofororbit/Wikimedia Commons.

The question I wondered about early on after that 2014 discovery was: does this transport of charged meteor dust between hemispheres explain, or at least contribute to, the propagation seen at upper HF/lower VHF?

Yesterday, years later, I got around to asking an expert about the atmospheric part (only).

Distinguished Professor Cora Randell is as much of an expert on things atmospheric as you are likely to find on this planet.  I came into contact with Cora some years ago concerning my interest in noctilucent clouds.

It's important to note that I did not ask Cora about radio propagation. My question was limited to only whether it was plausible that meteoric debris was in flux between the hemispheres of the planet.  

I was glad to learn that this was not an idiotic question!  Cora cited the results of a peer-reviewed paper of 2008 (Numerical simulations of the three-dimensional distribution of meteoric dust in the mesosphere and upper stratosphere, Bardeen et al, Journal of Geophysical research, Vol. 113, D17202, doi:10.1029/2007JD009515, 2008), summarising that:

"meteoric dust in January... is transported from the southern polar mesosphere and lower thermosphere into the northern polar atmosphere, and that just the opposite happens in July (when it is transported from the northern polar region to the southern polar region)." [MW1CFN - see Figure 3 in paper for details]

Again, there is no claim about radio propagation, because that is not what I asked, nor what Cora commented on.  

But it does confirm that charged debris is indeed in large-scale flux between the hemispheres of the planet.  There is therefore at least the possibility that this debris might contribute to upper HF/lower VHF radio propagation.

It's up to us, as mere radio operators, to now go and develop the ideas further.  As always, comments from 6m (and any other) experts welcome.  'Old man' criticism and angry nonsense is not.