venerdì 27 marzo 2020

Propagation News – 29 March 2020

March 27, 2020

With the solar flux index remaining at 70-71 all week, conditions were pretty predictable. The Kp index rose to two or three at times, but on the whole geomagnetic conditions were stable. There was a large coronal mass ejection off the Sun on 20 March, but it wasn’t Earth-directed and didn’t affect us.

It is at times like this when the slightest HF enhancement stands out. At 1030UTC on Wednesday, the 25th, the critical frequency as measured by the Chilton Digisonde rose to 5.675MHz, which meant a maximum usable frequency of more than 21MHz over a 3,000km path. But why did it rise?

NOAA issued a warning that the greater than 2 MeV electron flux reached high levels on Wednesday, with a peak flux value of 1,398 particle flux units. At about this time the GOES-16 satellite showed a fall in the X-ray flux coming off the Sun. Solar X-rays from the Sun penetrate to the bottom of the ionosphere to around 80km and create an enhancement of the D-layer. So, it could be that the lower X-ray flux meant less D-layer absorption and, coupled with the increased electron flux, HF signals were able to more easily reach the now-enhanced F2 layer.

The moral of this story is that HF propagation isn’t all about sunspots! If anyone did work any interesting DX on the higher bands around 1030UTC on Wednesday, please email the details to

Next week, NOAA predicts the solar flux index will remain at around 70. Geomagnetic conditions are predicted to be settled with a Kp index around two. This weekend may have unsettled HF conditions due to a large solar coronal hole,

VHF and up:

The recent high pressure has certainly looked promising on the weather maps, but as hinted last week, the air near the surface is very dry, and since good tropo needs moist air near the surface, overlain by warmer dry air above, we have not seen the bands live up to expectations based on the ‘hall barometer’ alone.

This high is being nudged west over the Atlantic, which will allow a cold northerly flow down across the country with a few wintry showers in the east at first, but maintain dry, high-pressure weather for most of the week. This means that there is a continuing chance of tropo, but don’t expect too much of it!

There has been some sporadic E on 10m and 6m. The primary driver of this appears to be related to an east-north easterly jet stream over the continent at just the right distance from the UK to support paths to south-east Europe and round to Italy and Spain. This favourable jet stream will decline by this weekend, but next week a further northerly jet stream may offer some chances, this time towards Scandinavia and eastern Europe.

The spring aurora period is upon us, so it is always worth checking for activity if the Kp index shows a disturbed magnetic field. The Moon reaches maximum declination on Wednesday and with just over a week to go to perigee, path losses will fall all week. 144MHz sky noise is at its lowest next weekend. (