The 2023-2027 General License question pool inquires about factors effecting propagation of HF signals:
G3A10: What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
A. Long term oscillations in the upper atmosphere B. Cyclic variation in the Earth’s radiation belts C. Rotation of the Sun’s surface layers around its axis D. The position of the Moon in its orbit
As I draft this article, the 11-year solar cycle is ramping up cycle 25, with increasing sunspot numbers that are significantly higher than the official predictions. A few years ago, Spot AR2192 shown in the image here was equivalent in area to 33 planet earths in surface area and could be clearly seen by the naked eye when solar illumination was attenuated by smoke, cloud, or fog. [For the record, Ham Radio School does not promote, advocate, or even slightly recommend staring at the sun under any viewing conditions. Bad idea almost anytime.]
So, what’s the big deal with sunspots and ham radio?
Although sunspots are magnetic regions of relatively cool temperatures that form the darker “spot” in appearance, they are brimming with ultraviolet radiation. The more sunspot area on the face of the sun, the greater the UV rays reaching earth. More UV rays passing through our atmosphere will produce more ions and make the ionosphere denser. A dense ionosphere does a great job of bending some RF signals back toward earth, improving over-the-horizon propagation and communications around the world. So, in short, greater sunspots usually mean enhanced ham radio fun. When the sun it very active with spots, the higher HF bands such as the 10-meter band will open for long-distance communications and provide tons of fun with very little power required. The density of contacts reported in the DXmaps image below gives you the idea.
The image here is of self-reported contacts on the 10-meter band within a 15 minute period. The green lines are 10-meter F2 ionospheric layer skip contacts, and the red lines are shorter 10-meter sporadic-E skip contacts. This provides some insight into the nice propagation that high sunspot activity can provide.
But the conditions won’t last forever. Sunspots vary over time and, as this question points out, HF propagation conditions will vary along with the sunspots. Two primary cyclical activities impact the sunspot variations.
11 year sunspot cycle: The sun is a complex and dynamic star, and its magnetic activity that drives sunspot creation varies naturally over an 11 year period. Sunspots will increase during the “solar maximum” and decrease near the “solar minimum.” The sun is currently moving toward a solar maximum expected in mid-2025.
Solar Rotation: Like the earth, the sun rotates on its axis. The period of surface layer rotation is about 28 days. As the sun rotates the sunspots will pass across the solar disk slowly and eventually rotate out of earth view. Long-lived and large sunspots will often return to again pass across the earth-facing hemisphere of the sun after having rolled around the back side over a couple of weeks’ time.
The answer to General Class question G2B10, “What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?” is “C. Rotation of the Sun's surface layers around its axis.”
-- Stu WØSTU