Which of the following signal paths is most likely to experience high levels of absorption when the A-index or K-index is elevated?
Why The A-index and K-index are measures of geomagnetic field disturbance, and disturbances are driven by charged particles funneled down the Earth's magnetic field lines into the high-latitude auroral zones. When those indices climb, particle precipitation ionizes the lower ionosphere (D region) over the auroral oval, so HF signals crossing polar or near-polar paths are strongly absorbed and may black out entirely. Lower-latitude paths are largely unaffected by the same event.
Watch out Transequatorial paths are the tempting opposite: they run through the low-latitude region and are governed by the equatorial anomaly, not by geomagnetic storm absorption, and can actually stay usable while polar paths are dead.
A and K are geomagnetic numbers, and geomagnetic trouble lives at the poles: high K, dead polar paths.