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

*Correct Answer is  A: Increasing disturbance of the geomagnetic field*  

### AI Response: (may contain errors)  

The correct answer to the question about the A-index or K-index is:

**A: Increasing disturbance of the geomagnetic field.**

### Explanation of the Correct Answer:
- The **A-index** and **K-index** are measures of geomagnetic activity.
- A rising A-index indicates that the geomagnetic field is becoming more disturbed, which is often caused by solar activity such as solar flares or coronal mass ejections.
- This disturbance can affect radio wave propagation, particularly at high frequencies.

### Why the Other Choices are Incorrect:
- **B: Decreasing disturbance of the geomagnetic field**  
  - This is incorrect because a rising index indicates an increase in disturbance, not a decrease.

- **C: Higher levels of solar UV radiation**  
  - While solar activity can lead to increased UV radiation, the A-index and K-index specifically measure geomagnetic activity, not solar UV levels.

- **D: An increase in the critical frequency**  
  - The critical frequency is related to the ionosphere's ability to reflect radio waves, which can be influenced by solar activity, but a rising A-index does not directly indicate an increase in critical frequency.

### Useful Formulas or Rules of Thumb:
- The A-index is a daily average of the K-index values, which range from 0 to 9, where:
  - 0 indicates very quiet conditions,
  - 9 indicates extreme geomagnetic storm conditions.
  
- A general rule of thumb is:
  - Higher A-index or K-index values correlate with poorer HF radio propagation conditions due to increased ionospheric disturbances.

Understanding these indices is crucial for predicting radio propagation and planning communications effectively.