*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 associated with solar activity, such as a coronal mass ejection (CME) or a solar flare. Here's why:

1. **Correct Answer (B): A coronal mass ejection impact or a solar flare has occurred**
   - Solar flares and CMEs release a large amount of energy and charged particles into space. When these particles reach Earth, they can cause increased ionization in the ionosphere, leading to heightened radio noise across the HF spectrum. This is because the increased ionization can enhance the scattering and reflection of radio waves, resulting in more noise.

2. **Incorrect Choices:**
   - **A: A temperature inversion has occurred**
     - Temperature inversions affect VHF and UHF propagation more than HF. They can cause ducting, which enhances signal strength over long distances, but they do not typically cause a sudden rise in HF noise.
   - **C: Transequatorial propagation on 6 meters is likely**
     - Transequatorial propagation is a VHF phenomenon, primarily affecting the 6-meter band (50 MHz). It does not cause a sudden rise in noise across the HF spectrum.
   - **D: Long-path propagation on the higher HF bands is likely**
     - Long-path propagation refers to signals traveling the long way around the Earth. While it can enhance signal strength, it does not cause a sudden increase in background noise across the HF spectrum.

In summary, the sudden rise in HF noise is a direct result of increased solar activity, which affects the ionosphere and radio wave propagation. Understanding these effects is crucial for predicting and interpreting radio conditions.