Open any propagation report and you will see two numbers that seem to say the same thing: the solar flux and the sunspot number. Both rise when the Sun is active and fall when it is quiet, so it is tempting to treat them as interchangeable. They are related, but they are measured differently, and for planning your operating day one of them is usually the better choice. Here is how to tell them apart.
What each number is trying to tell you
HF propagation depends on how strongly the Sun ionises the upper atmosphere, especially through its extreme ultraviolet and X-ray output. We cannot easily measure that output from the ground, so we use stand-ins, or proxies. Solar flux and sunspot number are two such proxies. Neither one is the thing that actually changes your signal. Each is a convenient clue about it.
Solar flux: an instrument reading
The solar flux index is the Sun's radio noise at a wavelength of 10.7 centimetres, which is about 2800 MHz. It is quoted in solar flux units. It is measured with a radio receiver, and the long-running daily measurements are made in Penticton, British Columbia. Because it is a radio measurement, it does not depend on clouds or on anyone's eyesight. It also never falls to zero: even in the quietest part of the solar cycle it sits at a baseline, roughly in the high 60s or low 70s. In strong years it can climb well past 200.
Sunspot number: a count with history
The sunspot number comes from counting dark spots on the Sun's visible surface. The count weights groups more heavily than individual spots, then applies a correction for each observer. Sunspots have been recorded for centuries, which makes this number invaluable for studying the solar cycle over long periods. The international version is compiled from many observers by the Royal Observatory of Belgium. Because it starts with human counting, it is more sensitive to observing conditions and to how spots are grouped. Unlike flux, it can read zero.
How they relate, and where they part ways
Over months and years the two track each other closely. The trouble is at the day-to-day level. A cluster of spots can rotate out of view, and the count changes, while the Sun's radio emission changes more gently. Flux is also affected by more than spots. Bright regions of the Sun that are not dark spots contribute to it. That is part of why flux is often the better match to ionospheric behaviour.
Also note that the flux you see is sometimes reported as observed and sometimes as adjusted. The observed value changes slightly through the year as the Earth-Sun distance changes, and the adjusted value corrects for that. When you compare readings from different sources, make sure you are comparing the same kind.
Which one to trust for day-to-day planning
For band planning, lead with the solar flux. It is an objective measurement, it is published consistently, and it tends to follow the ionosphere's response more smoothly than the raw daily sunspot count. Keep the sunspot number as background, especially the smoothed value, which is useful for understanding where we are in the cycle. If you want a wider view of the current cycle, our article on why Solar Cycle 25 is far from over puts the trend in context.
A rough flux guide by band
These are approximate rules of thumb, not promises, and local conditions can override them:
- Flux near the floor (roughly 70): the lower HF bands and 20 metres do most of the long-distance work. The highest bands are often quiet.
- Moderate flux (roughly 100 and up): 17 and 15 metres tend to open more often and for longer.
- High flux (roughly 150 and up): 12 and 10 metres have a better chance of wide daytime openings.
Even with low flux, the higher bands can still surprise you through other mechanisms such as sporadic-E, so a low number is a reason to listen rather than to stay off the band.
What neither number can tell you
Flux and sunspots describe the Sun's output, not the state of the path between you and another station. They say nothing about geomagnetic disturbance, which is what the A-index and K-index describe. A high flux with a disturbed geomagnetic field can still give poor conditions. We cover that side in what the K-index really means for your HF contacts. Time of day, season, your antenna and the location of the other station matter just as much. A solar flare can also cause a sudden loss of signals on the sunlit side of the Earth, and it can briefly inflate a flux reading.
A simple daily routine
- Check today's solar flux and whether it is rising or falling compared with recent days.
- Check the K-index and A-index to see if the geomagnetic field is calm.
- Choose a band that suits the time of day on the path you want.
- Open the live HF propagation page to see what conditions look like right now.
- Listen first. Real signals on the band beat any forecast.
Common mistakes
- Treating one number as a verdict. A single value is a clue, not a forecast for your particular path.
- Ignoring the geomagnetic indices. Good flux does not rescue a disturbed day.
- Judging a band by the sunspot count alone. A low daily count can still coincide with usable conditions.
- Mixing observed and adjusted flux. They are not directly comparable.
- Skipping the listening test. If the band sounds dead, it may be dead. If it sounds alive, call.
For a walk through reading a full forecast, see how to read a propagation forecast before you turn on the rig.
Build your own experience
The best way to learn what the numbers mean for your station is to compare them with your own results. When you log a contact in the free online logbook, note the band and time, then look at what the solar flux was that day. Over a few weeks you will see which bands work for your antenna at which values. That personal record is more valuable than any general rule. Remember that transmitting requires the correct licence for your country, and that the rules and band plans differ between Canada and the USA, so check the ones that apply to you.
The short version
Use solar flux as your daily planning number, keep the sunspot number for the big picture, and pair both with the geomagnetic indices and a live look at the bands. If you want the essentials on one page while you plan, the ham radio quick reference lists bands, modes and Q-codes.
How HamRadioList can help
Once you know which number to watch, check it against what the bands are actually doing. The HamRadioList live HF propagation page shows current conditions before you call CQ, and the free online logbook lets you record what you worked so you can compare your results with the numbers over time.
- Live HF propagation - see current band conditions before you call CQ.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Ham radio quick reference - bands, modes, phonetics and Q-codes on one page.
- Live operators - listen to amateur radio operators broadcasting live, with audio and an S-meter.
- Ham radio FAQ - plain answers to the questions new operators ask most.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.