Propagation News – 11 May 2025

Active region 4079 turned out to be not quite so bad as we had predicted. The sunspot was large, about ten times the width of our Earth, so we expected it to be more active. It was regions 4081 and 4082 that produced most of the flare activity last week.

Two filament eruptions also occurred within two hours of each other. The first event began at 16:20UTC on the 6 May in the southern hemisphere and was responsible for a plasma wave across the surface of the Sun.

The second filament eruption began in the northern hemisphere around 17:50UTC to the north east of AR 4079. A large amount of plasma was flung to the north. The filament eruptions on Tuesday 6 May did produce coronal mass ejections, but an Earth-directed component was not apparent. So we dodged a bullet!

The solar wind speed remained high for most of last week which didn’t help propagation. It did help to push the Kp index to five across the weekend of the 3 and 4 May. Things were more settled later in the week. On Thursday the 8 May the solar wind speed was down to around 500 kilometres per second, the Kp index was three, but MUFs over a 3,000km path were still down at about 20MHz.

An Earth-facing coronal hole may add to the solar wind this weekend, the 10 and 11 May. It looks like summer HF propagation is kicking in, with lower MUFs during the day but higher ones at night.

Next week, NOAA predicts that the SFI will be in the range 155 to 165. Settled geomagnetic conditions are forecast for the beginning of this coming week but unsettled geomagnetic conditions are expected for the 16 to the 19 May. At that point we could see the Kp index rise to five again, coupled with poorer propagation and reduced MUFs.

VHF and up 

Tropo has been a key propagation mode during the previous couple of weeks, but there have been variations in the location and timings of these conditions.

Sometimes this will be due to increased moisture below the temperature inversion, so although the cloudier areas have been cool, they have probably done better for Tropo.

This is due to the improved quality of the inversion, which performs better when there is a big moisture contrast across it, as you find with these layers of cloud.

Otherwise, we find the better conditions are driven by night-time cooling, forming a low inversion near the ground which tends to disperse during the morning as the Sun warms it away. This should still be beneficial for the 70cm UK Activity Contest on Tuesday 13 May over eastern areas, but perhaps less so farther west.

Good conditions will last into next week, but not everywhere. The main change will see an area of showery rain, possibly thundery, moving north into western Britain this weekend and perhaps some central areas early next week. This raises the prospect of some GHz band rain scatter in the west.

The Eta Aquariids meteor shower peaked last week. In the decaying tail of activity, together with other lesser showers, this should keep an enhanced meteor input for the coming week.

The Sporadic-E season typically runs from May to mid-September and the daily jet stream blogs have started on www.propquest.co.uk.

As usual, the main season offers two periods of activity, one in the morning and the second from late afternoon to the evening. Remember Es is not guaranteed since it depends upon many factors, but jet streams help, and these are shown on the website maps. Use the map clusters to find out where any activity is starting on 10m and then follow it up in frequency through the lower VHF bands.

EME path losses are at their maximum but falling after apogee on Friday 9 May. Moon declination is negative, reaching minimum this coming Thursday, so we’ll have very short Moon windows and low peak elevation. 144MHz sky noise is moderate, increasing to high by next Friday.

Category: GB2RS Propagation News