I notice several groups of 5MHz operators working each other around sunset, waiting for the skip to fade. Stations close to each other fade out first followed by those further apart. The more northerly paths will fade-out earlier that the southerly paths. It's fascinating to experience this, especially if you haven't studied it closely before. It's also quite surprising how quickly signals on a particular path can vanish.
However, I note that some people keep a close eye on the local sunset times and have grey-line maps available, and seem to be expecting that there is something 'magic' about the precise time at which the sun sets. This is all wrong.
The propagation mode in use on 5MHz around the UK at sunset is almost certainly via the F layer, which is at a height of approximately 300km. For a particular path, the relevant part of the F-layer is that which is above the midpoint of the path. If sunset has ANY significance at all in this matter, then it's going to be sunset at this point on the F-layer, not sunset on the ground.
If the sun is just setting on the ground, it will still be shining at a height of 300km, and a few quick calculations shows that the terminator (the grey-line) will be some 1800km displaced from the ground-level terminator. F-layer sunset will be more than an hour later than it is on the ground. If we were really concerned about precise sunset times, we should calculate it for the height of the ionosphere, not at sea level.
However, this isn't the whole story. At ground level we are familiar with the fact that it gets dark suddenly as the sun drops below the horizon and this visual phenomenon would still apply at the height of the F-layer. However the ionisation level doesn't drop suddenly when sunlight vanishes. Free electrons hang around for hours at this height and there's a slow decay in the critical frequency, not a sudden drop. Study of the FXi figures on the Chilton or Fairford ionosonde websites will confirm this. Indeed the critical frequency starts to drop several hours BEFORE sunset. This is simply because of the shallower angle - a given amount of solar radiation is spread over a larger area of the ionosphere as the sun moves further away from the zenith.
All of which brings me to the point of this posting. It's true to say that the sun sets later in summer and it's also true to say that 5MHz stays open later in summer, so it does make sense to run tests like this progressively later as the months go by but there is no significance in the precise timing of ground-level sunset.
73
Peter, G3PLX
