Explain the radio horizon, how flares and CMEs disturb HF, and what Bz, K and A indices, VOACAP and reporting networks tell you.
Syllabus: Propagation prediction and reporting: radio horizon; effects of space-weather phenomena
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Atmospheric refraction bends VHF/UHF slightly downward, so the radio horizon is about 15 percent farther than the geometric horizon. Both grow with the square root of antenna height.
RememberRadio horizon: about 15 percent farther.
A solar flare sends X-rays that reach us at light speed and cause short-term radio blackouts. A CME (a cloud of charged particles) arrives later. A sudden rise in HF background noise points to a CME impact or a solar flare.
Flare classes run A, B, C, M, X, each 10 times stronger than the one before.
RememberFlares cause short-term blackouts. X is the greatest class.
Bz is the north-south strength of the interplanetary magnetic field. Southward Bz links up with Earth's field and lets solar-wind energy in, so disturbed conditions become more likely.
A rising A-index or K-index means increasing geomagnetic disturbance. The extreme storm on the NOAA scale is G5. Paths through the auroral oval suffer the most absorption.
RememberBz south: trouble. K and A rising: more disturbance. G5: extreme.
Reporting networks collect digital-mode and CW signals actually heard. VOACAP models HF propagation. 304 Å is UV emission, tracking the solar flux index.
RememberHeard: reporting networks. Predicted HF: VOACAP. 304 Å: solar UV.