Propagation News – 4 August 2024

We had another week of high solar flux but unsettled geomagnetic indices.

The solar flux index, or SFI, was above 200 from the 27 July onwards. There was a worry that multiple coronal mass ejections, or CMEs, might push the Kp index up during the week. In the end, the Kp index didn’t rise above 5. But why was that?

The first of several CME eruptions was expected to pass Earth on the 29 July. The solar wind speed increased from 330 kilometres per second to around 450 kilometres per second, but only a brief period of minor G1 geomagnetic storming was observed. The solar wind stream was fast, at more than 450 kilometres per second, but its frozen-in magnetic field was predominantly North-facing or neutral. So, it didn’t really couple with the Earth’s magnetic field, and we ended up with a Kp index of only 5.

If we had had a south-facing interplanetary magnetic field or Bz it could have been much worse.

As it was, the ionosphere was affected on Wednesday 31 July. However, by 1830UTC things were back to normal, with a critical frequency of 7.4MHz, which gives a maximum usable frequency over a 3,000km path of around 21MHz.

Further disturbances on Thursday 1 August saw the Kp index rise to 5 again, but it was short-lived and only lasted for one three-hour period.

So, we are still in a period of high solar activity, with numerous M- and even X-class flares, coronal mass ejections and high sunspot numbers. With a cluster of sunspots rotating into view right now, it should be an interesting week ahead!

The SFI is predicted to fall next week, perhaps to the 180s or 190s. Meanwhile, HF propagation remains good at times, although we are still feeling the effects of the summer doldrums, with reduced daytime maximum usable frequencies.

Keep an eye on solarham.com for up-to-date geomagnetic information.

The 15m band is still the favoured daytime DX band, with occasional F2-layer openings on the 12m and even 10m bands. However, most 10m openings are still due to Sporadic-E.

VHF and up

The weather patterns that drive some of our radio propagation conditions are in the process of changing.

As we ended the previous week, there were still regions of high pressure near enough to southern areas for some useful Tropo across the North Sea, English Channel and south over Biscay.

The slow change towards low pressure dominating the charts will be gradually extending its influence over the whole country as we move into next week, so less Tropo and more interest in rain scatter.

This has been available over some northern areas recently and from heavy thunderstorms in the south. It is probably going to be a useful mode for exploring the GHz bands in the coming week, particularly where heavy thundery rain occurs.

Meteor scatter is also a worthwhile pursuit since we are still in the broad period of the Delta Aquariids meteor shower and that eventually blends into the August Perseids later in the month.

The solar conditions continue to be challenging and the Kp index has been high enough to make it worth thinking of checking for auroral conditions – either fluttery signals on the LF bands overnight, or perhaps auroral conditions on the VHF bands. The Kp index gets mentioned on all propagation websites these days, but spaceweather.com is a good source for that and much else too.

Sporadic-E is still worth checking out and, although the summer jet stream pattern is usually pretty weak, it is still there, and the position is perhaps a bit more northerly now than earlier in the summer. This means that paths towards the east to Scandinavia and the Baltic might be worth looking for, although other directions can still happen, of course.

Moon declination started to fall from Friday 2 August, so Moon window lengths and peak Moon elevation will follow suit. With apogee on Thursday 8 August, path losses will be at their highest for the month. 144MHz sky noise is low until the evening of Saturday 3 August when we see the Moon and Sun close in the sky until moonset on Sunday. After that, the sky noise is low.

Category: GB2RS Propagation News