The sun runs the ionosphere

Everything on HF starts with the sun. Its ultraviolet and X-ray output builds the ionosphere; its solar wind and eruptions can tear it apart. A few numbers tell you most of what you need to know.

The numbers that matter

• Solar flux (SFI, 10.7 cm flux): a daily measure of solar activity. Below 80 is low, 100–150 is good, above 150 is excellent for the high bands.
• Sunspot number: rises and falls with the 11-year cycle, and tracks closely with solar flux.
• Kp index: geomagnetic disturbance on a 0–9 scale, updated every three hours. 0–2 is quiet; 5 and up is a storm.
• A index: a daily average of the same disturbance. Under 10 is quiet; over 30 is stormy.
• Solar wind speed and Bz: fast wind (over about 500 km/s) with Bz pointing south (negative) for hours is the recipe for a geomagnetic storm.
• X-ray flux: solar flares, graded A, B, C, M and X, each ten times the last. M and X flares cause sudden daytime HF fadeouts.

NOAA’s three storm scales

• R (radio blackouts, R1–R5): caused by flares. HF fades on the sunlit side of the earth, from minutes to a few hours.
• S (solar radiation storms, S1–S5): energetic particles cause polar cap absorption, wiping out HF paths over the poles, sometimes for days.
• G (geomagnetic storms, G1–G5): the ionosphere is disturbed, HF is noisy and patchy at high latitudes, and aurora can bring unusual VHF openings.

Why it matters more in Southern Alberta

At nearly 50° north, we feel geomagnetic storms sooner and harder than stations farther south, and our paths to Europe and Asia cross high latitudes. The upside: aurora is closer, and a big storm can bring 6 m and 2 m aurora contacts that southern stations never get.

Where to go next

• PropMap: all of these numbers, live
• NOAA Space Weather Prediction Center (swpc.noaa.gov)
• Propagation Fundamentals

Last updated October 2026.