Solar Cycles and Band Conditions
The sun drives HF propagation. The amount of ultraviolet and X-ray radiation the sun produces determines how strongly the ionosphere is ionised, which in turn determines which HF bands are open, to what distances, and at what times. The sun's activity follows a roughly 11-year cycle between solar minimum and solar maximum — a cycle that has shaped amateur radio operating patterns for over a century and remains the dominant factor in long-term band conditions.
What drives the cycle
The sun's magnetic field undergoes a regular reversal — approximately every 11 years, the north and south magnetic poles of the sun swap. During the buildup to this reversal, magnetic activity at the solar surface increases dramatically, producing sunspots — dark, cooler regions on the sun's surface where intense magnetic field lines prevent convection. Sunspots are associated with increased ultraviolet and X-ray emission from the surrounding active regions. This increased radiation is what ionises the upper atmosphere and enhances HF propagation on the higher frequencies.
Solar maximum vs solar minimum
At solar maximum, sunspot numbers are high (regularly above 100–150 on the daily smoothed sunspot number), the Solar Flux Index routinely exceeds 150–200, and the 10m, 12m, and 15m bands support worldwide DX propagation daily. 6m occasionally opens to transcontinental F2 propagation — a rare event that only occurs near solar maximum. At solar minimum, sunspot numbers drop to near zero for extended periods, SFI falls below 70, and 10m and 12m may be completely closed for months. 20m and 40m remain useful throughout the cycle.
| Index | What It Measures | Good Value | Poor Value |
|---|---|---|---|
| Solar Flux Index (SFI) | 10.7cm radio emission — proxy for ionisation | Above 150 | Below 70 |
| Smoothed Sunspot Number (SSN) | Average daily sunspot count over 13 months | Above 100 | Below 20 |
| A-index | Daily geomagnetic disturbance (0–400) | Below 10 | Above 30 |
| K-index | 3-hour geomagnetic disturbance (0–9) | 0–2 | 5 or above |
| X-ray flux | Solar X-ray emission level | A or B class | M or X class flares |
Solar flares
Solar flares are sudden explosions on the sun's surface that release enormous amounts of X-ray radiation. When a major flare occurs on the sunlit side of Earth, the increased X-ray flux reaches Earth in about 8 minutes (at the speed of light) and dramatically increases D-layer ionisation on the sunlit side. This causes a Shortwave Fadeout (SWF) — a sudden, complete absorption of HF signals on all sunlit paths. A major X-class flare can cause a complete HF blackout lasting minutes to hours. Paths in darkness are not affected. SWFs are most common during the years around solar maximum when flare activity is highest.
Coronal Mass Ejections (CMEs)
A Coronal Mass Ejection is a large cloud of magnetised plasma ejected from the sun. Unlike flare radiation which travels at the speed of light, CME plasma takes 1–3 days to reach Earth. When a CME arrives, it interacts with Earth's magnetic field and causes a geomagnetic storm — disrupting the ionosphere and degrading HF propagation, sometimes severely. A strong geomagnetic storm (K-index 5–9) can close the higher HF bands for days and produce aurora visible at unusually low latitudes. The HF disruption from a CME can last 1–3 days as the storm runs its course.
Where we are now
Solar Cycle 25 began in December 2019 and has been significantly more active than the weak Solar Cycle 24 that preceded it. Cycle 25 reached solar maximum in 2024–2025 with sunspot numbers and solar flux levels that exceeded initial predictions, producing excellent conditions on 10m, 12m, and 15m for the first time in over a decade. Operators who had never experienced a strong solar maximum were treated to daily worldwide contacts on 10m — a band that had been essentially dead for years during Cycle 24's weak maximum.
Planning around the cycle
Understanding where we are in the solar cycle helps you plan your operating strategy. Near solar maximum, invest time in the higher HF bands — 10m and 12m contacts are spectacular and require less antenna investment than lower bands. Near solar minimum, focus on 40m and 20m which remain viable throughout the cycle. For equipment purchases, a modest antenna at 10m is worthwhile near solar maximum but provides little value at solar minimum. DXCC chasers specifically target the years around solar maximum for new entities on 10m and 12m.
How do I find the current solar flux index?
The NOAA Space Weather Prediction Center at swpc.noaa.gov publishes real-time solar data including SFI, A-index, and K-index. The WWV time signal (2.5, 5, 10, 15, 20 MHz) broadcasts current solar flux, A-index, and geomagnetic conditions at 18 minutes past each hour. DX propagation websites like DXWorld.net and hamqsl.com aggregrate current solar data for quick reference. Most amateur radio propagation apps display current solar indices prominently.
Does higher solar flux always mean better conditions?
Higher solar flux generally means better conditions on the higher HF bands (10m–20m), but it is not the only factor. A high SFI combined with a disturbed geomagnetic field (high K-index from a geomagnetic storm) can actually produce poor conditions despite the high solar activity. The combination of high SFI and low K-index produces the best HF conditions. Additionally, the time of day, season, and specific propagation path all affect conditions regardless of the solar flux level.
Will 10 metres ever be as good as it was in past solar cycles?
Solar Cycle 25 has already shown that 10m can be spectacular again — the band has been regularly open worldwide during the 2024–2025 peak. Each solar cycle is different in timing and intensity, but 10m consistently provides remarkable propagation at solar maximum. Operators who missed previous solar maxima should take full advantage of the current peak while conditions are favourable, particularly on FT8 where the mode's sensitivity allows contacts on marginal openings that phone operators would miss.