G3A10 from the General License Course Section 3.1, Solar Activity:
What causes HF propagation conditions to vary periodically in a 28-day cycle?
A. Long term oscillations in the upper atmosphere
B. Cyclic variation in the Earth’s radiation belts
C. The Sun’s rotation on its axis
D. The position of the Moon in its orbit
As I draft this General Class Question of the Week, the largest sunspot in 25 years is passing across the face of our sun. Spot AR2192 is equivalent in area to 33 planet earths in surface area and can be clearly seen by the naked eye when solar illumination is attenuated by smoke, cloud, or fog. [For the record, HamRadioSchool.com 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. Presently, the 10-meter band is wide open due to AR2192 and its pals, providing amazing performance with the higher HF bands and opening 6-meter sporadic E.
The image here is of self-reported contacts on the 10-meter band within a 15 minute period today. 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 we are currently experiencing [Late October 2014].
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 near the solar maximum .
Solar Rotation: Like the earth, the sun rotates on its axis. The period of 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.
You can learn more about solar dynamics and the impact on radio propagation here on earth in this previous Ham Radio 101 article, Sunspots and Propagation.
The answer to General Class question G2B10, “What causes HF propagation conditions to vary periodically in a 28-day cycle?” is “C. The Sun’s rotation on its axis.”
Related Questions: G3A01, G3A09, G3A11