How to read the propagation indices: SFI, SSN, K and A
Every conversation about HF conditions comes down to four numbers: SFI, SSN, the K index and the A index. They get passed on the air, printed in forecasts and written in logs — but not everyone can read them.
In short: the first two say how much the Sun is giving out (and therefore how dense the ionosphere is), and the last two say how quiet the Earth's magnetic field is (and therefore whether a storm is choking that ionosphere).
Below is a full account of each index: what it means physically, how to read it, what it affects and when it is worth getting on the air. Beside each section there is a live gauge that pulls the current value from NOAA and shows where we stand.
Note that the colours run in opposite directions: SFI and SSN turn green upwards (more light, better conditions), while K and A turn green downwards (quiet is better for the low bands). That is not a whim but physics, and it is explained below.
Two pairs of indices
It helps to hold all four in your head as two pairs, because they answer two different questions.
Solar (what the Sun is giving):
- SFI — the Sun's radio flux at a wavelength of 10.7 cm;
- SSN — the number of spots on the solar disc.
These decide how dense the F layer of the ionosphere is, and therefore whether the high bands (10–20 m) will open.
Geomagnetic (what is happening at the Earth):
- K index — the highest geomagnetic activity over 3 hours;
- A index — the average geomagnetic activity over a day.
These decide whether a magnetic storm will blow that ionosphere away, and therefore whether the low bands (40–160 m) hold up.
The ideal condition for HF is a high SFI and a low K. Now each index in turn.
SFI — the solar flux index
What it is. The solar flux index is the power of the Sun's radio emission at 2800 MHz (a wavelength of 10.7 cm), measured by the observatory at Penticton, Canada, and published by NOAA. The unit is the sfu (10⁻²² W·m⁻²·Hz⁻¹). It is a convenient proxy for ionising ultraviolet: the brighter the Sun shines at 10.7 cm, the more it ionises the F layer.
Hence the main rule: the higher the SFI, the denser the ionosphere and the higher the ceiling (the MUF — the highest frequency the ionosphere will still reflect).
The current SFI, right now:
How to read it:
- 60–80 (red, “poor”) — solar minimum. The high bands (10–15 m) are dead; 20–40 m work, and even then unreliably.
- 80–100 (yellow, “middling”) — moderate activity. 20 m works decently, 15 m opens from time to time.
- 100–250 (green, “good”) — a high SFI. 10, 12 and 15 m open, and long DX paths on HF come alive.
What it affects. Chiefly the high bands (10–20 m). There the SFI decides whether there will be a contact at all. On the low bands (40–160 m) the SFI matters little — there it is geomagnetic quiet that rules.
In practice: once the SFI reaches about 100 it is worth trying 10 and 15 m; at 150–200 you can count on transcontinental paths on the high HF bands all day long.
SSN — the sunspot number
What it is. The sunspot number (the Wolf number) counts the groups and the individual spots on the solar disc, weighted by size. Spots are regions of heightened magnetic activity, and they radiate more ionising energy.
Why it tracks the SFI. Spots and the 10.7 cm emission are both products of solar activity, and they are related almost linearly (a correlation above 90 %). So reading the SSN largely repeats the SFI, but it states the number of spots on the Sun directly, which many people find more vivid.
The current SSN, right now:
How to read it:
- 0–30 (red, “minimum”) — solar minimum. The high bands barely work.
- 30–60 (yellow, “moderate”) — activity rising; 15–20 m are steady.
- 60–250 (green, “high”) — an active Sun, good HF conditions, frequent flares and storms.
What it affects. The same as the SFI: the density of the F layer and the high bands. The SSN swings through the eleven-year solar cycle: at the peak it can run into the hundreds, at the minimum it falls almost to zero.
In practice: watching the SSN tells you where in the eleven-year cycle we are — whether activity is rising or falling.
The K index — geomagnetic activity over 3 hours
What it is. The K index measures the disturbance of the Earth's magnetic field as seen by individual observatories. The planetary Kp averages them worldwide. The scale is logarithmic, from 0 (completely quiet) to 9 (an extreme storm), in thirds of a point.
Every 3 hours NOAA issues a new Kp — the fastest pulse of the geomagnetic state you get.
Why it matters on HF. A magnetic storm disturbs the lower ionosphere (the D layer) and chokes propagation on the low bands. The higher the K, the worse conditions get, especially on 40–160 m and on polar paths.
The current K index, right now:
How to read it:
- 1–3 (green, “quiet”) — ideal. The low bands are at their best.
- 3–6 (yellow, “unsettled”) — moderate disturbance. 40 and 80 m are somewhat worse; on the high HF bands it is barely noticeable.
- 6–9 (red, “storm”) — a magnetic storm. 40–160 m are heavily suppressed, aurora is possible, and polar paths collapse entirely.
In practice: Kp ≥ 5 is a signal that the low bands will be poor; Kp ≤ 2 is the best time for 40, 80 and 160 m.
The A index — geomagnetic activity over a day
What it is. The A index is a daily roll-up of the K index. Each three-hourly K is converted into an “equivalent amplitude” (a), and the eight intervals of the day are then averaged. The result is a linear scale from 0 to a few hundred, which compares one day with another more conveniently than the logarithmic K.
How to read it:
- 0–40 (green, “quiet”) — the magnetosphere is in good shape, HF is normal.
- 40–100 (yellow, “unsettled”) — disturbed; the low bands are noticeably worse.
- 100–400 (red, “storm”) — a strong storm; 40–160 m are effectively closed.
The current A index, right now:
What it affects. The same as the K index, but on a daily scale. The difference is speed: K catches the short shove of a storm, A shows what the day was like as a whole.
In practice: the A index is useful when going back over logs — it makes it easy to see which day was geomagnetically quiet.
Two kinds of index, two directions of colour
Look at the zone scales once more and remember the central symmetry:
| Index | A high value | Colour cue | What a high value means |
|---|---|---|---|
| SFI | Good (more light) | green at the top | nothing — always a plus |
| SSN | Good (more spots) | green at the top | nothing — always a plus |
| K index | Bad (a storm) | red at the top | a magnetic storm |
| A index | Bad (a storm) | red at the top | a magnetic storm |
The practical conclusion: the best conditions come when the SFI is high and the K is low. That combination is what gives you open high bands and working low ones at the same time.
Where the data comes from
The live gauges and all the zone thresholds take their data from NOAA's open sources:
- SFI and SSN — the daily table
daily-solar-indices.txt; - K index and A index — the series
noaa-planetary-k-index.json(updated every 3 hours).
The zone boundaries for the SFI (green begins at 100) and for the K index (quiet from 1 to 3) follow the way these indices are usually shown on DX clusters.
What to watch next
Once these four numbers make sense, so does a propagation forecast:
- Look at the SFI: if it is 100 or more, try the high bands (10–20 m).
- Look at Kp: if it is 2 or less, take on 40–160 m without hesitation.
- Compare the A index with the day before: if it has doubled or tripled, the magnetic background is deteriorating — wait.
- When going through a log, remember: the number of contacts depends more on the calendar (weekends) than on the Sun, whereas the high bands depend on the SFI exactly as described.