Propagation News – 25 May 2025

A high-speed solar wind, originating from a massive coronal hole, took its toll on HF this past week.

During the weekend of the 17th and 18th, the Kp index was above four for nine of the three-hourly recording periods. Unfortunately, the feed from the Dourbes Digisonde failed over this period, so we can’t comment on what effect this had on maximum useable frequencies (MUFs).

Later in the week, geomagnetic conditions improved, despite the coronal hole still being Earth-centric, and the Kp index fell to more reasonable levels, sometimes in the ones and twos.

An M3.2 solar flare was observed at 0821UTC on 19 May. The source was located off the north-east limb, so perhaps we can expect more as it turns to become more Earth-facing.

Unfortunately, the solar flux index also fell to lower levels, reaching 119 by Thursday, the 22nd. In fact, on that day it had been below 120 for more than seven days. But all is not lost as a solar flux of more than 100 is usually enough to keep the HF bands open.

MUFs over a 3,000km path have generally reached 21MHz and have often peaked at 24MHz at times. It is quite normal for the F2-layer MUF not to reach 28MHz due to a change in the ionospheric chemical make-up in the summer—the so-called summer doldrums. However, sporadic E may keep 10m alive so don’t write the band off.

Propagation, at the moment, favours paths into the southern hemisphere, although each band can have different characteristics at different times. A quick play with the propagation tool at tinyurl.com/propcharts will show you what we mean.

Next week, NOAA predicts that the solar flux index will remain around 120 until 27 May, after which it could rise to 140 by the end of the month. Unsettled geomagnetic conditions are forecast for the period from 28 May to 2 June. This could see the Kp index peak at five or six with corresponding drops in the MUF and DX.

As we are now on the downward portion of this solar cycle, we may expect more geomagnetic disturbances from coronal holes and coronal mass ejections.

VHF and up propagation news from G3YLA and G4BAO

The long run of fine settled weather seems to have drawn to a close.

We have a period of changeable and, at times, wet and windy weather to deal with over the coming week. I doubt we will see much, if any, tropo except perhaps towards the end the week; as some models show a new high building as we move into June.

Before then, the various fronts and showers will bring several spells of rain, and a much greater chance of rain scatter on the GHz bands than recently.

The solar conditions have again provided weak auroral events, with fluttery signals on the HF bands being a good indicator, since the bright evenings and early dawns leave little chance of any visual clues. As usual, the measure to follow is the Kp index. So, look out for values of Kp going above five to generate interest.

Meteor scatter is declining as the last of the Eta Aquariids play out, following a long tail from the peak on 6 May. Meteors also make up the content of sporadic E because, as they burn up, they produce long-lived metallic ions that can be focused into thin layers of sporadic E.

There are some less well-known meteor showers in June. So, next week there may be more to say. In the meantime, random meteors and declining Aquariids will have to suffice.

Now, on to sporadic E. This is beginning to be the primary mode in these summer weeks. It is sensible to check during main operating windows in the morning, late afternoon and early evening.

Ideally, we require meteor input, a low Kp index, plus some weather triggers to set up atmospheric gravity waves which produce the right wind shear in the E region. Jet streams are usually the best clues for where sporadic E might form.

Use the daily sporadic E blog on Propquest to get an insight of the current day’s weather patterns.

If you have very limited time and have still not worked sporadic E, just checking 50MHz in the late afternoon will ensure that you make a start this year.

EME path losses are still falling as the Moon reaches perigee, its closest point to Earth, on 26 May. Moon windows and peak elevation are again rising, reaching a maximum on 29 May. 144MHz sky noise is low and remains low as the week progresses.

Category: GB2RS Propagation News