Propagation News – 24 March 2024

As we said last week—what a difference a month makes. Only in this case, what a difference a week makes!

On Friday, the 15th, the solar flux index, or SFI, was at 129 and the Sun did not look very spotty at all. This week it’s all changed. On Thursday, the 21st, the solar flux index stood at 176 with seven active regions visible.

For HF propagation forecasts, the golden rule is to use smoothed sunspot numbers (SSN), rather than a daily figure—that is, sunspot numbers averaged over the past 12 months.

The current SSN is 129, which equates to a solar flux index of about 144. Perhaps the daily figure of 176 has the psychological effect of giving us confidence that the bands will be open!

A moderately strong M7.4 solar flare was observed around active region 3615 at 0736UTC on 20 March. The region continues to rotate into a better Earth-facing position and will remain a threat for additional solar flares above the M5.0 threshold.

If a flare leads to a coronal mass ejection (CME), we could expect a disturbed ionosphere roughly 48 hours later, depending upon the solar wind speed.

Also, a large coronal hole on the solar equator became Earth-facing on Thursday, the 21st. A coronal hole is a lower-energy area on the Sun with open magnetic field lines that lets solar plasma escape. This may lead to a geomagnetic disruption this weekend, ending this Sunday, the 24th, with increased Kp numbers and a lowering of the maximum usable frequency.

Next week, NOAA predicts that the SFI will be in the 155 to 165 range. Geomagnetic conditions are hard to predict, due to coronal hole effects and CMEs, but NOAA puts the Kp figure at two to three. If we get a CME, this could easily rise to five or more.

Finally, we are now at the equinox which means that auroras are more likely due to the Russell-McPherron effect. The equinox is also a good time for north-south HF paths. Also, note that the HF bands are staying open for longer with the 4U1UN beacon in New York, the YV5B beacon in Venezuela, and the 4X6TU beacon in Israel audible on 14.100MHz around 2230UTC last week.

VHF and up propagation news from G3YLA and G4BAO

It’s not good news for weather-related propagation with a total absence of high pressure on the charts again and a resulting lack of tropo for the majority of the coming week.

But tropo is not the only mode to be affected by the weather, rain scatter is too.

This has been a player quite often recently and, as we move into the spring, the characteristic April showers can be very useful for rain scatter. Their effectiveness is related to the sixth power of the raindrop diameter, so the larger drops in heavier April showers can be increasingly valuable on the GHz bands. Many good online weather radar displays are available to allow you to track them for maximum benefit.

Other promising mentions should go to aurora which, as we said earlier, can be more likely around this time of the year, when the Earth’s magnetic field is more favourably coupled to the solar wind. Meteor scatter is a lesser option and rests with random chance activity, as we are still some way off from any major meteor showers.

Sporadic E and related transequatorial propagation (TEP), in the shorter term during the next month or so, will soon be a major player. For TEP between Europe and southern Africa, check clusters around the middle part of the day. Although usually favouring stations in southern Europe, it can occasionally turn up for UK operators, particularly for those located in the southern half of the country.

As for sporadic E, the new season is starting to wake up, so the usual maxim of checking clusters is the best advice, starting with the 10m band and working up to the 6m band. The traditional morning and late afternoon windows are less reliable in early-season events, so just check anyway if you’re in the shack.

For EME operators, Moon declination is still positive, but going negative again this coming Monday. Path losses are at their maximum due to apogee on Saturday, the 23rd. 144MHz sky noise is low but rising to moderate by the end of the coming week, reaching more than 1,000 Kelvin on Sunday, the 31st.

Category: GB2RS Propagation News