In my previous post, I gave a few links mentionning the "high angle mode". I found extremely interesting G0KYA's paper "the twilight zone".
This is the introduction: Steve Nichols G0KYA, of the RSGB's Propagation studies Committee, believes that propagation around sunrise and sunset is not fully understood. Here he outlines the mechanisms behind grey line and other twilight propagation modes and a research project to help us understand them.
And a particulary interesting paragraph Now imagine a radio wave hiting the ionosphere at about 75-85 degrees to the earth - a near vertical incidence wave (NVIS). Below the critical frequency, it would be returned. At some frequency close to fof2 it could be refracted through a large angle and could end up travelling almost parallel to the earth, giving a very long first skip distance. This is the condition for the Pedersen or critical ray, discovered in 1927, characterised as being high angle, long distance and close to and probably above the fof2 frequency. As there would be no intermediate ground hops the signal strength could be very high indeed.
Following this reading, I exchanged a few emails with Steve (one of them, reproduced here with his authorization)
Hi Patrick, Thank you very much for your e-mail. I have had a quick look at your blog and it is very interesting. Your message was actually very timely as I have been looking at WSPR as a greyline beacon tool, but had put it to one side to concentrate on other things - I will pick it up again! Here are some thoughts and observations that might be useful.
1. VOACAP is not very good at predicting openings below about 5MHZ. It was never intended for this and is usually very pessimistic. W6ELProp is better.
2. You are quite right - it doesn't take into account sunrise enhancements I did some work on signals from VP6DX into the UK on 80m and using VOACAp looked at the number of modes (hops). It showed clearly that there are more modes around sunrise and the radiation angle goes higher. I can send you this if you want. I am therefore convinced that greyline has more to do with higher angle signals due to ionospheric tilting and multiple modes than the old "D layer not illuminated/F layer illuminated" model.
This also explains why you get good greyline openings at right angles to the terminator.
Thanks for thinking of me.
RSGB Propagation Studies Committee
You will find other articles and powerpoint presentations on Steve page, definitely a "must read" for whoever is interested by those phenomenas (the above pictures are from Steve presentations). Steve's VP6DX presentation is available as a PDF.
By the way, the "high angle mode" has a name: The Pedersen Ray... and nothing new, since it was discovered in 1927...