Tides, winds, and atmospheric gravity waves
In the page on ablated meteors and plasma, I discussed how ionisation from meteors works. Now I’ll move on to discuss the core process of node formation.
As I noted at the end of the page on ablated meteors and plasma, the ionisation produced from the basic process of meteor entry is insufficient to support VHF propagation – and certainly insufficient to cause the excitement of Es.
Something special is needed to densify the ionosphere at about 100km up. Something else is needed to explain how E region propagation comes about from normal low electron density E region layers.
And, as I’ve hinted, it’s all about nodes.
Descending plasma
The starting point in node production is a stable but descending plasma above 125km.
You’ll recall that the E region of the ionosphere is at about 100km up. That’s where the sporadic VHF propagation action is!
The region above about 125km is riven with winds caused by differing air pressures as the sun cooks the atmosphere differently with time and location.
Winds move the plasma east and west at differing heights. The result is a tearing interaction or shear where the adjacent air flows meet. And that shear traps the plasma in tidal nodes of enhanced density. Those densified nodes continue to descend into the E region with time.
As you might imagine, this core process of node formation alone would create a stable state with a stable refractive layer or layers. But the formation and maintenance of tidal nodes varies over ground features like mountains and oceans. Ground features cause upwardly propagating gravity waves. These gravity waves propagate into the E region and above and they move the descending tidal nodes back and forth. So, this modifies the core process of node formation to give descending, continuously mobile, high electron density nodes.
Node density
The descending densified nodes are not all the same size and density. Size and density are determined by:
- the cooking of the plasma by the sun with time (ionising the atoms);
- the effectiveness of node formation (by winds and tides); and,
- the speed of node descent.
As I’ve intimated elsewhere, size and density are greatest in summer.
The result of this dynamic node development is a normal situation that enables E region openings at VHF. Those occur, or should occur in the early morning, afternoon from mid-day to about 9pm, and after midnight around 3am.
I’ve summed this up in the above diagram.
I’ve described openings as diurnal, day diurnal and night diurnal (with diurnal just meaning daily). Remember that times are at the refraction point – which is about two hours ahead of UK time when looking for an opening to the east. The effect is most pronounced in the mid latitudes.
Add in some chaos
This core process of node formation is not the whole story.
The situation that I’ve described so far would see openings at the same time each day. Hams operating Es know that there can be a strong opening one day, with nothing the next.
Simply, several phenomena from time to time insert chaos into the otherwise stable structure. Thunderstorms, high levels of variation in the Earth’s geomagnetic field (signified by high Kp) and change in the jet stream of upper air all influence. As a result of these effects, nodes can be easily dispersed, decimating their refraction capability.
So, to conclude, let’s bring those stable descending nodes and some chaos together. Whilst you might enjoy a VHF opening one day, you can never say if a similar opening will occur the next. Simply, the core process of node formation yields a stable structure – that is then decimated from time to time by chaos. As I’ve described here, the resulting instability makes E region VHF propagation ‘sporadic’.
I describe those chaotic mechanisms on the next page.