Friday 23 October 2009

Making sense of 10m propagation

What's this? G stations working VK on 10m at sunspot minimum? Jordan and Thailand audible on 10m on a 65ft end fed?

That can't be right!

Well, it looks like it is. So why did 10m suddenly open up this week?

I think we need to look at a number of factors. First the solar flux was 72 so obviously no massive increase in solar UV output has caused it.

But it is October and this is a peak month for DX. The ionosphere in the Northern Hemisphere is cooling down, becoming more dense and the chemical composition changes too in the Winter compared with Summer, giving us a greater ion density.

To quote J K Hargreaves' “The Solar Terrestrial Environment”: You get an increase in NO concentration and a decrease ion loss rate due to the removal of water-cluster ions and a change in chemical reaction rates due to the temperature change.

In fact, if you feeling really awake read the whole 414 pages – I am sure the more I read the less I understand!

Anyway, so is that the whole story? No!

At the time of the openings the he Bz (Interplanetary Magnetic Field) was pointing south which means that solar wind was coupling with the earth's magnetic field and hot ions were pouring in.

Hence the 6m aurora the previous night – see

The three-hourly K index reached 4 showing the effects of these ions on the earth's magnetic field.

So what we probably saw was a pre-auroral enhancement and that condx on the upper HF bands while good, are going to get worse. I'll let you know if I was right.

And they say that a high K index is bad for propagation – not always at the outset though.


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    Image by NASA

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