*Question E3C01:
What is the cause of short-term radio blackouts?*  

*Correct Answer is  D: Solar flares*  

### AI Response: (may contain errors)  

Short-term radio blackouts are primarily caused by solar flares. Here's why:

1. **Correct Answer: Solar flares (D)**
   - Solar flares are intense bursts of radiation from the sun. When they occur, they emit a large amount of X-rays and ultraviolet radiation, which can reach Earth in about 8 minutes. This radiation can ionize the Earth's upper atmosphere, particularly the ionosphere, leading to increased absorption of radio waves. This results in short-term radio blackouts, especially affecting high-frequency (HF) communications.

2. **Why the other choices are incorrect:**
   - **Coronal mass ejections (A):** While coronal mass ejections (CMEs) can cause geomagnetic storms and auroras, they are not the primary cause of short-term radio blackouts. CMEs take longer to reach Earth (1-3 days) and primarily affect the Earth's magnetic field rather than causing immediate ionospheric disturbances.
   - **Sunspots on the solar equator (B):** Sunspots themselves do not directly cause radio blackouts. They are regions of reduced surface temperature on the sun and are associated with solar activity, but they do not emit the radiation that causes immediate ionospheric disturbances.
   - **North-oriented interplanetary magnetic field (C):** The orientation of the interplanetary magnetic field (IMF) can influence geomagnetic activity, but it does not directly cause short-term radio blackouts. A north-oriented IMF is generally less likely to cause geomagnetic disturbances compared to a south-oriented IMF.

3. **Useful Information:**
   - Solar flares are classified by their intensity, with X-class flares being the most intense. The impact on radio communications is often proportional to the intensity of the flare.
   - The ionosphere is crucial for HF radio propagation, and any disturbance can significantly affect radio communication.

Understanding the role of solar flares in causing radio blackouts is important for predicting and mitigating their impact on radio communications.