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Propagation News – 14 February 2021

At the risk of sounding like a stuck record, last week’s HF propagation was characterised by zero sunspots and the geomagnetic effects of a coronal hole. The lack of sunspots is now getting a bit worrying, especially when you consider that traditionally the new cycle normally ramps up quite quickly. At least one joker has said that perhaps we had sunspot maximum for Solar Cycle 25 back in December 2020!

A high-speed stream from a pair of Earth-facing coronal holes pushed the Kp index up to five in the early hours of Sunday 7th February. This was back down to one by Monday, but conditions remained subdued for a time. With the solar flux now back at 74, and the Australian Space Weather Services T index at six, it doesn’t bode well for HF propagation right now. The T index is the sunspot number equivalent that best matches the observations made by ionosondes.

Next week, NOAA predicts the solar flux index will remain around 73. It also predicts the Kp index will be two. However, a large polar-connected coronal hole became Earth-facing on Thursday, which suggests we may get an elevated Kp index some time across the weekend. So look for a pre-auroral enhancement as the solar wind speed increases and check out 10 metres for any potential openings.

Daytime Maximum Usable Frequencies over a 3,000km path are currently exceeding 18MHz and often more than 21MHz at times. There is still evidence of mid-winter short-skip Sporadic-E paths occurring, with Scottish stations coming into England on 18MHz on Thursday.

The F2 region critical frequency, or f0F2, is still falling quite quickly after sunset. Eighty metres is largely closing to inter-UK near-vertical incidence skywave contacts by about 2000hrs, as contestants in last week’s 80m Club Championship found out. The only good news is that spring is coming and with it hopefully an upturn in HF propagation.

VHF and up:

Intense cold over a snow field is again a theme for a while and although there is a slow trend for a gradual warm up from the west, the high pressure over the North Sea and northern Europe will probably take a lot of shifting. This means that there continues to be a possibility of limited temperature inversions due to the high itself and over the snowy ground in eastern Britain and thus Tropo is a limited possibility. Weather fronts encroaching into western Britain may introduce limited precipitation scattering options.

The big question is whether the milder air actually gets in properly, since highs like these are very stubborn and may hold fronts to the west of the UK. The models are suggesting that this time the colder air to the east may well be the winner here and might not give way or, if it does, should reassert itself next week. So keep your Tropo options open, especially over eastern areas.

Moon declination goes positive again on Monday so visibility windows and peak Moon elevations will increase. Apogee is on Thursday so path losses are at their highest. 144MHz sky noise is low but increasing to 500K next Sunday.

There are no significant meteor showers until mid-March, so stick to the pre-dawn period for the best random meteor scatter conditions.

And that’s all from the propagation team this week. (rsgb.org)

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