6m, Es to JA, and gravity waves.

Following on from a brief series of posts about summertime paths on 6m from Europe to JA, I undertook a mini-literature review on what science understands already.

I started off by thinking about E-layer detection by ionosonde.  That's because I didn't properly understand the detail.

I was very intrigued - but not surprised - to find a paper published only in March 2019, told of how Es intensity derived from ionosondes has been incorrectly assessed - for decades - because somehow, the entire community 'forgot' that a major component of Es layers is metallic ions!

As well as giving a crucial reminder that metallic ions - which are sourced from meteoric debris precipitating from higher up in the atmosphere - are important, it also shows how much we always have to check what we 'know' is actually not based on a basic mistake, made long ago.

I am no expert on 6m propagation, but it seems pretty clear to me that if you have a layer of metallic ions at the E height of around 100km (slightly above the mesopause), then those ions are going to move around in three dimensions fairly easily.   What we need to know is what mechanisms give rise to that movement.

Well, the usual cause that most texts on 6m Es propagation will give you is: 'wind shear'.  There's nothing wrong with this mechanism, and is neatly explains one way in which opposing wind directions lead to metal ions being concentrated in a thin layer.  Most scientists agree this is a correct model.

But wind shear is not the only mechanism.  We also have gravity waves.  These are simply waves in three dimensions that propagate upwards from the lower atmosphere - caused by various physical disturbances near the ground - and reach the mesopause, where they become unstable and 'break', just like a water wave, except in air and without the complication of two different media.

I then found, accepting it's fairly old by now, a professional scientist writing in the ARRL's QST magazine about what was, to him, the puzzling fact that Es has a strong seasonal bias.  His thoughts took him down what I can only really describe as a mental black hole.

To his credit Dr. Whitehead, unlike many folk, understood very well the critical importance of metal ions to Es.  But he then tries to explain the seasonality by suggesting that the metal ions must change in abundance with the time of year.  He then comes up with a rather convoluted mechanism that purports to explain this change.

Waves in a noctilcuent cloud layer, 2009.  The way sunlight is reflected off the waves' peaks shows how radio (also electromagnetism, rather obviously) could also propagate selectively off these surfaces.

But the phenomenon of gravity waves needs no such convolution of thought.  Their occurrence peaks in the few weeks either side of midsummer.  This is simply because the summer hemisphere's surface warms up strongly and quickly, leading to upward transport of enormous parcels of air.  This, by simple expansion as the pressure drops, leads to cooling.  By the time the parcels have reached the 'lid' of the mesopause, the temperature in summer at around 85km is a frigid minus 125 degrees Celsius or so.

Gravity waves just add somewhat to this background cooling.  They are kicked up by winds over mountains, thunderstorms causing local energy inputs, and even the other Es mechanism - wind shear.  Even infrasound, caused by ocean waves, is often given as one mechanism likely causing gravity waves.

As these waves break, they lead to localised extra cooling.  Often, they are the sole reason how the mesosphere is tipped just over the frost point, so that visible noctilucent clouds form.  Without the gravity waves, NLC - or at least visible NLC - might not have formed on some nights.

So, back to 6m summertime propagation to JA, and such far-flung regions.  First, credit to IK3XTV who, as long ago as 2008, published his thoughts on QSL.net about the role of gravity waves from thunderstorms in radio propagation.  I think we have to find IK3XTV was a considered analyser of what was going on, even if he was only really thinking about gravity waves from storms, and not all other mechanisms.

PMSE ('ice') occurs not in a single, narrow layer, but across several km in height.

What, then, do we know?

(1) We have a layer of metallic ions which become coated with charged ice in summer that occur roughly at E height.  This height is not fixed, but varies by many kilometres.

(2) The mesosphere (about 15km below E height), is largely quiescent in winter, when there is little energy to disturb it from below, but highly disturbed in summer.

(3) These disturbances are caused by - and form - large-wavelength wave structures at the mesopause.

(4) Windshear is not the only mechanism that can concentrate ions.  Gravity waves also can.

From these three things, we can pretty confidently suggest that it's very likely the waves present rapidly-varying reflective (or refractive, if you prefer) surfaces from which 6m signals can be propagated. 

The fact that 6m propagation to JA is transient, and not permanent during summer is simply because it requires the correct alignment of different areas of 'wavy structures' to allow a signal to hop successfully from Europe to JA.  This alignment, because the wind velocity at 85km is up to 400m/s, is never likely to last for very long for any given two stations.  But the occasional, continuous background of mass upward movement of gravity waves can give rise to several hours of conditions that will allow such propagation.

Altogether, this mechanism for explaining summertime Es propagation at 6m seems entirely non-controversial and simple.  But responses to JE1BMJ's article have tended to bring about a dismissal from other operators who think they know better.  This dismissal seems simply to have been yet another manifestation of people - often a bit older - who have become rigid in their thinking, coupled to not actually knowing very much about the atmosphere they rely upon.

But there's plenty we can do to shore-up the evidence.  Es propagation isn't limited to 6m, and my own interest with 12m, for which I'm much better equipped, shows it has a big influence there, too.  Over the past few days, I can see that, when there is NLC and/or PMSE present, I will get FT8 signal reports around midnight from at least the distance of Europe when beaming to the NNE.  On nights when there are no NLC or PMSE, it appears those signal reports do not occur, or are much more limited.

In the end, should we be be surprised if a layer of metal ions, with some charged ice as well, and disturbed in these complex and beautiful ways by waves, sometimes gives rise to 6m DX?  We may not be able to prove (yet) that any given QSO is due to this mechanism, but, in general, I think it will explain most such QSOs.

Where I will yield is that there is plenty more evidence to gather, and thought to give!