Question E3C02:
What is indicated by a rising A-index or K-index?  

Choices:  
A: Increasing disturbance of the geomagnetic field  
B: Decreasing disturbance of the geomagnetic field  
C: Higher levels of solar UV radiation  
D: An increase in the critical frequency    

Correct Answer is  A: Increasing disturbance of the geomagnetic field  

Correct answer: A — Increasing disturbance of the geomagnetic field

Why A is correct:
- The K-index is a quasi-logarithmic measure (0–9) of 3-hour geomagnetic activity; higher K means more magnetic field disturbance.
- The A-index is a daily, linearized version of the K-index (approximately 0–400). A rising A or K indicates stronger geomagnetic storms driven by solar wind/CME interactions with Earth’s magnetic field.
- Practically: rising A/K often means degraded HF propagation (depressed MUF, especially on polar paths) and increased auroral absorption; sometimes enhanced VHF auroral propagation.

Why the other choices are incorrect:
- B: Decreasing disturbance of the geomagnetic field — opposite of what a rising A/K means.
- C: Higher levels of solar UV radiation — UV/EUV ionization is tracked by the Solar Flux Index ($F_{10.7}$), not A/K. High $F_{10.7}$ generally improves HF by raising foF2/MUF, independent of geomagnetic disturbance.
- D: An increase in the critical frequency — Geomagnetic storms (high A/K) typically depress the F-region, lowering foF2 and MUF, not increasing them.

Useful rules of thumb:
- K-index thresholds:
  - $K_p \le 3$: quiet
  - $K_p = 4$: unsettled
  - $K_p \ge 5$: geomagnetic storm (G1+)
- Relationship cues:
  - Rising $A$ or $K \Rightarrow$ more geomagnetic disturbance, worse HF (especially polar), possible aurora.
  - Rising $F_{10.7} \Rightarrow$ more ionization, higher foF2 and MUF (often better HF), independent of A/K unless a storm follows.