*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)  

A rising A-index or K-index indicates increasing disturbance of the geomagnetic field. Here's why:

1. **Correct Answer Explanation (A):**
   - The **A-index** and **K-index** are measures of geomagnetic activity. They reflect the level of disturbance in the Earth's magnetic field caused by solar activity.
   - A rising index means that the geomagnetic field is becoming more disturbed, often due to solar storms or increased solar wind activity.

2. **Why Other Choices Are Incorrect:**
   - **B: Decreasing disturbance of the geomagnetic field**  
     - This is incorrect because a rising index indicates more disturbance, not less.
   - **C: Higher levels of solar UV radiation**  
     - The A-index and K-index do not measure solar UV radiation. They specifically measure geomagnetic disturbances.
   - **D: An increase in the critical frequency**  
     - The critical frequency relates to ionospheric conditions and radio wave propagation, not directly to geomagnetic disturbances.

3. **Useful Information:**
   - The **K-index** is a logarithmic scale from 0 to 9, where higher numbers indicate more geomagnetic disturbance.
   - The **A-index** is a linear scale derived from the K-index, providing a daily average of geomagnetic activity.

Understanding these indices helps radio operators anticipate conditions that might affect radio wave propagation, especially at higher frequencies.