*Question E3C12:
Which of the following is indicated by a sudden rise in radio background noise across a large portion of the HF spectrum?*  

*Correct Answer is  B: A coronal mass ejection impact or a solar flare has occurred*  

### AI Response: (may contain errors)  

A sudden rise in radio background noise across a large portion of the HF spectrum is typically indicative of increased ionospheric activity, which is often caused by solar events.

### Correct Answer: 
**B: A coronal mass ejection impact or a solar flare has occurred.**

- **Explanation**: 
  - Coronal mass ejections (CMEs) and solar flares release large amounts of energy and charged particles into space. When these events reach Earth, they can enhance ionization in the ionosphere, leading to increased background noise across HF frequencies. This noise is often heard as static or interference on the bands.

### Why the Other Choices are Incorrect:

- **A: A temperature inversion has occurred**  
  - **Explanation**: A temperature inversion affects radio wave propagation by bending signals, but it does not typically cause a rise in background noise. Instead, it can improve signal quality by reducing fading.

- **C: Transequatorial propagation on 6 meters is likely**  
  - **Explanation**: Transequatorial propagation is a specific type of propagation that occurs under certain conditions, often related to the ionosphere's state. While it can be influenced by solar activity, a sudden rise in background noise is not a direct indicator of this type of propagation.

- **D: Long-path propagation on the higher HF bands is likely**  
  - **Explanation**: Long-path propagation refers to signals traveling a longer route around the Earth. While it can occur during certain ionospheric conditions, a rise in background noise is not a reliable indicator of this phenomenon.

### Summary:
- A rise in background noise is primarily associated with solar activity (CMEs or solar flares), which enhances ionospheric conditions.
- Understanding the effects of solar events on radio propagation is crucial for predicting changes in signal quality and noise levels.