Explain how solar activity changes HF conditions, how fast flares and particles arrive, and what SFI, K and A measure.
Syllabus: Sunspots and solar radiation; geomagnetic field and stability indices
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Sunspots mark an active Sun that emits more ultraviolet and X-rays. These ionize the F region more strongly, so it can bend higher frequencies back to Earth. Drag the slider.
At the cycle's low, 15, 12 and 10 m are the least reliable long-distance bands. 20 m supports daylight worldwide paths at any point. Zero sunspots does not kill propagation, it mainly weakens the highest bands. The Sun rotates in 26 to 28 days, so conditions repeat on that rhythm.
RememberHigh sunspots, high bands. 20 m works at any point in the cycle.
A flare's UV and X-rays travel at light speed: about 8 minutes. They ionize the sunlit side, adding absorption that hits lower frequencies hardest (a sudden ionospheric disturbance). Charged particles are slow. A coronal mass ejection takes 15 hours to several days. Coronal hole particles disturb HF.
RememberLight: 8 minutes. Particles: hours to days.
SFI is the Sun's radio noise at 10.7 cm, a measure of solar radiation. The K index is the short-term stability of Earth's geomagnetic field. The A index is the long-term one.
Memory hookK = short, A = long. Neither is a sunspot count.
A geomagnetic storm is a temporary disturbance of Earth's field. It degrades high-latitude HF paths. The same activity makes aurora, which can reflect VHF signals.
RememberStorm: polar HF down, aurora VHF up.