Tuesday, 18 August 2015
I've just made some changes to my monthly propagation charts.
I've added some more data sources so that you can see the current solar and geomagnetic conditions as well as the predicted HF coverage maps from the UK.
This means that you can get an at a glance look at likely HF propagation conditions – all on one page. The data include the current solar flux index, the Kp index and also solar wind characteristics.
We have been suffering from a number of recurring coronal holes recently (areas on the sun where the magnetic field is weaker, letting plasma out to form the high-speed solar wind).
If the magnetic polarity of this solar wind is “south” (we say that its Bz is pointing south) it is more likely to couple with the earth's magnetic field and the hot plasma can flood in.
The net result is the earth's magnetic field is distorted and we see this reflected in the Kp index, which normally rises - a geomagnetic storm is in progress.
The initial effects can be a short improvement in HF conditions, but these can be short lived. We then see an overall drop in maximum useable frequencies, the bands can get noisier and signals drop away, often with lots of heavy fading (QSB). It can take 24-48 hours for the ionosphere to recover, if it is not hit again.
The net effect is a lowering of overall critical and maximum useable frequencies as the plasma hits and excessive absorption, especially on polar paths.
We've seen a lot of this recently with poor conditions on HF.
So for good HF conditions look for settled geomagnetic conditions with a low K index for a day or so, a low solar wind speed (less than 450 km/s) with a Bz that is neutral or pointing north and a high solar flux index.
If you have a high Kp index, a high solar wind speed and a Bz pointing south don't be surprised if the bands aren't so good! This would not normally be seen in the monthly charts, which are an "average" for the month.
The charts are also now being carried on the RSGB web site too at http://rsgb.org/main/technical/propagation/hf-propagation/uk-short-path-monthly-forecast-maps/ which is why I have kept them long and thin to fit into the frame on RSGB.org.