Question E3C03:
Which of the following signal paths is most likely to experience high levels of absorption when the A-index or K-index is elevated?  

Choices:  
A: Transequatorial  
B: Through the auroral oval  
C: Sporadic-E  
D: NVIS    

Correct Answer is  B: Through the auroral oval  

Correct answer: Through the auroral oval

Why this is correct:
- Elevated A-index or K-index indicates geomagnetic disturbance. This drives energetic particle precipitation into the auroral oval, greatly increasing ionization and collision frequency in the D/E regions there.
- The result is strong “auroral absorption” (and during proton events, “polar cap absorption”), which severely attenuates HF signals crossing the auroral zone.

Why the other choices are not correct:
- Transequatorial (TEP): Involves F-region irregularities near the geomagnetic equator. It’s not primarily associated with high absorption during geomagnetic storms; TEP can occur independently and may even be enhanced by certain disturbances.
- Sporadic-E: Es is a reflective E-layer phenomenon that can open paths rather than absorb them. While geomagnetic activity can influence Es occurrence, it is not typically characterized by high absorption.
- NVIS: NVIS is more sensitive to D-layer absorption from solar X-ray/UV (daytime, flares). While storms can affect NVIS at high latitudes, the “most likely” severe absorption with high A/K is along auroral/polar paths, not NVIS in general.

Useful formulas and rules of thumb:
- Geomagnetic indices:
  - $K$: quasi-logarithmic, 0–9 (≥4 indicates a storm).
  - $A$: linearized average of $K$ (larger A = more disturbance).
- Ionospheric absorption roughly scales as:
  $$A \propto \frac{N_e \,\nu_c}{f^2}$$
  where $N_e$ is electron density, $\nu_c$ is collision frequency, and $f$ is frequency. Storms increase $N_e$ and $\nu_c$ in the auroral zone, boosting absorption, especially on lower HF.
- Rule of thumb: Avoid HF paths crossing auroral/polar regions when $K \ge 4$; choose lower-latitude great-circle paths.