Today’s Weather: 6000 degrees Celsius with a possibility of storms

HF propagation forecasts have always baffled me. A quick look at the forecast chart produced by Paul N0NBH on his excellent website at: hamqsl.com/solar.html reveals a dizzying collection of data from which Paul draws a conclusion about conditions on each of the HF bands. His forecasts are usually correct, but why?

I don’t have any inside secrets to reveal, but I did some research on what each of the data points tells us. I now have a better understanding and can view the “Solar-Terrestrial Data” with a modicum of wisdom. You may also be interested in learning a little more or sharing your own information on the topic.

It is quite important to have at least a basic understanding before climbing up to a SOTA peak or driving for hours to do a POTA activation. It could affect your decision on which antenna to take with you, or whether your QRP rig will have enough muscle get the job done. It might even convince you that today would be a good day to earn some credit with your significant other by getting through your chore list instead of playing radio!

Let’s take a look through some of the parameters in Paul’s Solar-Terrestrial Data chart.

Solar Flux Index (SFI)

The Solar Flux Index is a measure of radio intensity at 2800MHz originating from the Sun. It is measured in solar flux units (s.f.u.) and varies between around 50 to about 300. It is a good predictor of HF propagation on the higher bands – 20m and up. Generally, the higher the SFI the better. for more information visit: https://www.swpc.noaa.gov/phenomena/f107-cm-radio-emissions

Sunspot Number

Anybody who was tracking the Sunspot Number towards the end of solar cycle 24 will be aware that there were long periods with no sunspots at all. Now as we approach the peak of solar cycle 25 we see a sustained higher level of sunspot activity. Sunspots are caused by intense magnetic flux originating from deep within the Sun and appear as dark areas on the Sun’s surface. Higher sunspot counts are associated with better radio communications on Earth but can also be the source of solar flares and Coronal Mass Ejections (CMEs) that disrupt HF communications. The Sunspot Number is actually an index based on 10 times the number of visible sunspot groups.

A-Index

The A-index is an indication of the variation in the Earth’s magnetic field. It is measured on a scale of 1-400 and represents the daily average of the K-index. A high A-index may look bad for HF radio communications, but since it is calculated from the previous 24-hour period, the K-index provides a better short term indication of current conditions.

K-index

The K-index is an indication of the variation in the Earth’s magnetic field over a shorter term. It is updated every 3 hours. When designated as “Planetary K-index” it represents data originating from several monitoring stations around the world. A lower K-index is better for HF radio propagation. It is measured on a scale from 1-9 where an index below 5 is considered normal. Above 5 represents various levels of solar storm. If the K-index rises above 7 we may not be able to work DX but we might be able to enjoy low latitude auroral displays. More information is available at: https://www.swpc.noaa.gov/products/planetary-k-index

Whoa! If the K-index is measured on a scale of 1-9 how can the A-index ever reach a value greater than 9? The very best explanation I could find came from a ham, Kevin VE3EN. To summarize, the K-index is non-linear and is first converted to a linear scale where a K-index of 9 becomes 400. The A-index is then the average of 8 three-hourly linearized K-index values. I highly recommend reading Kevin’s excellent explanation at: https://www.solarham.net/a.htm.

X-Ray Flux

The X-Ray Flux is “a biggie” when it comes to its effect of HF radio communications. Solar flares in the X-ray part of the spectrum are detected by the GOES satellites in geostationary Earth orbit. Solar X-rays hitting the Earth’s Sun-facing side disturb the ionosphere causing HF blackouts if strong enough. The strength of solar X-ray flares is measured in Watts/m^2 and the flares are categorized as A,B,C,M and X where A is the weakest and X the strongest. NOAA issues alerts when the intensity reaches M5 (5x10e-5 W/m^2). X-ray flux data is updated every minute and and so has a very important and immediate impact on radio communications. More information at: https://www.swpc.noaa.gov/products/goes-x-ray-flux

304 Angstrom Radiation

Ultra-violet light from the Sun at a wavelength of 304 Angstroms provides a clear visible picture of solar prominences that can be used to assess the potential impact of solar flares and Coronal Mass Ejections. Satellites are used to observe this wavelength. Study of this radiation assists in the preparation of solar weather forecasts but any short-term impact on HF radio communications is unclear.

Proton Flux

High energy protons emitted from the Sun are monitored primarily by the GOES geostationary satellites, although NOAA Low Earth Orbit (LEO) satellites also provide data. The principal effect is to increase ionization in the Earth’s D-layer which absorbs RF and inhibits F-layer skywave propagation. This data is updated every 5-minutes and so has a very important and immediate impact on radio communications. Detailed information is available at: https://www.swpc.noaa.gov/products/goes-proton-flux.

Electron Flux

Electron flux is also measured by the GOES satellites, but its principal impact – although serious – is on satellites where it can cause charge build-up inside the spacecraft. Its impact on HF propagation is unclear although some sources indicate it could increase noise in radio receivers. The Space Weather Prediction Center describes electron flux at very high altitudes well above typical F-layer height. More information at: https://www.swpc.noaa.gov/products/goes-electron-flux

Solar Wind

“Solar Wind” refers to the stream of emissions from the Sun. Its speed is not constant and large increases in solar wind speed cause disruptions in the Earth’s geomagnetic field up to and including geomagnetic storms. The effect on HF radio communications is often high noise level in receivers – such as static crashes. More information is available at: https://www.swpc.noaa.gov/products/real-time-solar-wind.

Conclusions

Let me declare up front that I am no expert. I have investigated this topic and read several sources of data. In compiling this information I may have misinterpreted some data and perhaps neglected other important sources. I strongly suggest that you follow the links provided and draw your own conclusions.

The intent of this post is to identify and share which factors I believe have the greatest impact on amateur radio operators planning a temporary field operation (POTA, SOTA etc) – especially when using QRP equipment. Maybe QRPp (<1 watt) is not a good idea today. Perhaps it would be better to leave that short loaded whip at home today and take a full-size wire antenna instead. The reverse may also be true. Today may be the perfect day to test out that new Elecraft KH-1 handheld HF QRP rig!

Although space weather enthusiasts may thrive on digesting the parameters we have discussed, amateur radio operators probably want to focus on the data that has a more immediate impact on their operations. For example, the Solar Flux Index has the greatest impact on 20m and above. That is where we find a lot of POTA activity as we near the peak of solar cycle 25. The A-index is yesterday’s data – who cares? The K-index tells us what conditions were like during the last 3 hours. The Proton Flux and the X-Ray flux have a very immediate impact measured in minutes and warrant careful attention if you are about to set off on a xOTA activation.

If you choose to ignore the HF propagation forecasts and just “go-for-it” while hoping for the best, you may get lucky and still make lots of contacts – or you may be met with an HF blackout. Good luck.

Errors, omissions and other comments are very welcome. Let me hear your thoughts!

Other sources I have found useful:

https://www.swpc.noaa.gov/impacts/hf-radio-communications

https://qrparci.org/resource/FDIM81.pdf

https://noji.com/hamradio/pdf-ppt/noji/Noji-Article-Band-Conditions-Banner.pdf

https://www.solarham.net

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