*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 a breakdown of the correct answer and the other choices:

### Correct Answer: D: Solar flares
- **Explanation**: Solar flares are intense bursts of radiation from the sun that can significantly increase the ionization in the Earth's ionosphere. This increased ionization can disrupt radio wave propagation, leading to short-term blackouts, especially on high-frequency (HF) bands. The effects can last from minutes to hours, depending on the intensity of the flare.

### Why the Other Choices Are Incorrect:
- **A: Coronal mass ejections (CMEs)**: 
  - While CMEs can cause long-term disturbances in the Earth's magnetosphere and can lead to geomagnetic storms, they typically do not cause immediate short-term radio blackouts. Their effects are more gradual and can last for days.

- **B: Sunspots on the solar equator**: 
  - Sunspots are areas of reduced surface temperature on the sun caused by magnetic activity. They are associated with solar activity but do not directly cause short-term radio blackouts. Instead, they are indicators of solar activity cycles.

- **C: North-oriented interplanetary magnetic field**: 
  - The orientation of the interplanetary magnetic field can influence geomagnetic activity, but it is not a direct cause of short-term radio blackouts. It is more related to the effects of solar wind and CMEs on the Earth's magnetosphere.

### Summary
- **Key Point**: Solar flares are the primary cause of short-term radio blackouts due to their immediate impact on ionospheric conditions.
- **Remember**: CMEs and other solar phenomena can affect radio communications but typically over longer periods and are not the direct cause of short-term disruptions.

This understanding will help you answer questions related to solar activity and its effects on radio communications effectively.