What changed
Space weather — edited Aug 31, 2026 by Unknown editor
This is the first recorded version, so every line is shown as added. Note: Imported from content/wiki
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[Propagation](/wiki/technical/propagation) on HF is driven almost entirely by the Sun. These are the
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numbers every cluster page and club net quotes, and what they actually tell you.
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## The numbers
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- **Solar flux index (SFI)** — radio noise from the Sun at 2800 MHz, a proxy for ionization. Below
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about 70 the high bands are dead; 100–150 opens 15 m and 12 m; above 200, 10 m and 6 m come alive
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worldwide. Higher is better, with no downside.
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- **Sunspot number (SSN)** — the visible count, closely tracking SFI. The 11-year solar cycle takes
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it from near zero at minimum to well over 150 at maximum.
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- **A index** — a daily average of geomagnetic disturbance, 0 to 400. Under 10 is quiet; over 20
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degrades the high latitudes and the polar paths.
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- **K index** — the same thing every three hours, 0 to 9, logarithmic. K of 0–2 is good, 4 is
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unsettled, 5 and up is a geomagnetic storm and the low bands go long-path or nowhere.
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The short version: **high flux and low A/K is what you want.**
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## Events
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- **Solar flares** — X-ray bursts that ionize the D layer on the daylit side and cause a sudden
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ionospheric disturbance: HF simply disappears for minutes to an hour. Classed A/B/C/M/X.
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- **Coronal mass ejections** — plasma arriving one to three days after a flare. They trigger
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geomagnetic storms, aurora, and absorption on polar paths; 160 m and 80 m suffer most.
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- **Coronal holes** — recurring high-speed solar wind streams, roughly every 27 days with the Sun's
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rotation. Mildly disruptive and quite predictable.
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- **Aurora** — bad for normal HF, excellent if you want to bounce VHF signals off the auroral
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curtain (rough, buzzy CW and SSB on 6 m and 2 m).
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## Where to look
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- NOAA Space Weather Prediction Center — the authoritative source for alerts and forecasts.
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- solarham.com and hamqsl.com — the familiar at-a-glance amateur summaries and widgets.
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- VOACAP Online — a predicted path between two locations for a given mode and power.
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- **PSK Reporter** and the **Reverse Beacon Network** — where your signal is actually being heard
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right now. This beats any prediction.
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## Using it
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Predictions tell you where to look; the beacon networks tell you the truth. If the flux is high and
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K is low, try the high bands in daylight and expect long grey-line paths at dawn and dusk. During a
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storm, drop to 40 m and 30 m, work regionally, and remember that the
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[weak-signal modes](/wiki/operating/weak-signal-and-scatter) still get through when voice cannot.