Solar cycle

Solar cycle
Length	About 11 years on average (roughly 9 to 14)
Tracked by	Sunspot number and 10.7 cm solar flux
Current cycle	Solar Cycle 25, which began in December 2019
Cycle 25 maximum	October 2024 (smoothed sunspot number about 161)
Daily indices
Solar flux index	The sun's radio output at 2,800 MHz; higher is better for HF
K index	0–9, geomagnetic disturbance over three hours; lower is better
A index	0–400, daily geomagnetic average; lower is better
Published by	NOAA Space Weather Prediction Center, among others

The solar cycle is the roughly 11-year rise and fall in the sun's activity, seen most easily in the number of dark sunspots on its surface. More activity means more ultraviolet radiation, a denser ionosphere, and higher frequencies that will carry signals over long distances. It is the biggest long-term influence on HF propagation: an antenna that works the world on 10 meters at solar maximum may hear almost nothing there at minimum.

Sunspots and the cycle

Sunspots are cooler, darker patches where intense magnetic fields break through the sun's surface, and the active regions around them are the source of solar flares. They have been counted systematically since the 1700s, and cycles are numbered from one that began in 1755. A cycle starts at solar minimum, when weeks can pass without a spot, climbs over three to five years to solar maximum, and then declines more slowly. Cycles vary in strength as well as length; Cycle 24, which peaked in 2014, was the weakest in about a century.

Solar Cycle 25 began in December 2019. It proved stronger than the official forecast, which had called for a peak smoothed sunspot number of 115. The Solar Influences Data Analysis Center (SIDC) at the Royal Observatory of Belgium, which maintains the international sunspot number, found that the cycle reached its maximum in October 2024, with a smoothed sunspot number (a 13-month running average) of about 161. By 2026 it was in its declining phase, with large flares and geomagnetic storms still possible over the next few years. NOAA's Space Weather Prediction Center says Cycle 26, which begins at the next solar minimum, is expected to start some time between January 2029 and December 2032, although it has not yet issued a forecast for that cycle.

For HF operators, the decline means that openings on 10 and 12 meters are likely to become shorter and less frequent from year to year, while 15 and 17 meters usually hold up somewhat longer. By the minimum, 10 and 12 meters will be mostly closed apart from sporadic E and occasional north–south paths, and 15 and 17 meters will be limited to short daytime openings plus sporadic E in summer, as in the table below.

Measuring activity

The sunspot number is ten times the number of spot groups plus the number of individual spots, adjusted for each observer and combined from many observatories. The international series was revised in 2015, so older books quote lower figures for the same activity.

The solar flux index (SFI) is the strength of the sun's radio noise at a wavelength of 10.7 cm (2,800 MHz), measured daily at an observatory in Penticton, British Columbia. It tracks the ultraviolet output that ionizes the F layer. A quiet sun reads about 65 to 70; as a rough guide, readings above about 150 mean 10 and 12 meters are open on most days. The trend over several days matters more than one reading.

The K index rates disturbance of Earth's magnetic field over a three-hour period on a roughly logarithmic scale of 0 to 9. The A index is a daily figure on a linear scale of 0 to 400, derived from the eight three-hour readings. Planetary versions, Kp and Ap, combine magnetometers around the world. For both, low is good: a K of 0 to 2 and an A in single digits mean quiet conditions, while a K of 5 or more marks a geomagnetic storm. NOAA's Space Weather Prediction Center publishes these numbers with forecasts, and many logging programs and websites display them.

Effect on each band

The higher the frequency, the more a band depends on solar activity.

Band	Near solar maximum	Near solar minimum
160, 80 and 60 m	Usable at night, with more absorption and storm noise	Often at their best, with low absorption and quiet conditions
40 and 30 m	Regional by day, long distance at night	Much the same; the main DX bands after dark
20 m	Open by day and often all night	Open by day, closing a few hours after dark
17 and 15 m	Open worldwide most days	Short daytime openings at best, plus sporadic E in summer
12 and 10 m	Worldwide openings, even with low power	Mostly closed apart from sporadic E and occasional north–south paths
6 m	Occasional F2 openings across oceans near strong peaks	Sporadic E every summer; F2 openings rare

Flares, CMEs and storms

Solar flares are sudden bursts of radiation, graded by X-ray output as class A, B, C, M or X, each ten times stronger than the last. The X-rays arrive in about eight minutes and boost absorption in the ionosphere's D layer across the daylit side of Earth, causing a radio blackout: HF signals fade out, lowest frequencies first, for minutes to a few hours. NOAA rates blackouts from R1 (minor) to R5 (extreme). The night side of Earth is unaffected.

A coronal mass ejection (CME) is a huge cloud of magnetized gas thrown off by the sun. If one hits Earth, usually one to three days later, it can trigger a geomagnetic storm, rated G1 to G5. Storms deplete the F2 layer and lower the maximum usable frequency; polar paths fail first, and HF can stay poor for a day or more. Fast solar wind from coronal holes causes milder disturbances that tend to recur every 27 days, the time the sun takes to rotate as seen from Earth.

Storms have an upside. They push the aurora toward lower latitudes, and VHF operators can bounce signals off it on 6 and 2 meters, with a distinctive rasping tone. The severe storm of May 2024, the strongest in about two decades, brought auroras deep into the southern United States.

Check the K index before blaming your antenna. If it is 5 or higher, a geomagnetic storm is under way, and empty bands are the sun's doing, not yours.

See also

• Radio propagation
• Ionosphere
• DXing
• 10-meter band