*Question E3A13:
Which of these emission modes is best for auroral propagation?*  

*Correct Answer is  A: CW*  

### AI Response: (may contain errors)  

Auroral propagation is a unique mode of radio wave propagation that occurs when signals are reflected off the aurora borealis. This type of propagation is characterized by rapid signal fading and distortion due to the irregular and rapidly changing nature of the aurora.

### Correct Answer: CW

1. **Why CW is Correct:**
   - **CW (Continuous Wave)** is the best mode for auroral propagation because it is highly resilient to the rapid fading and distortion caused by the aurora. CW signals are narrowband, which means they can be more easily distinguished from noise and distortion. The simplicity of Morse code, used in CW, allows operators to decode signals even when they are weak or distorted.

### Why Other Choices are Incorrect:

2. **SSB (Single Sideband):**
   - SSB is a voice mode that is more susceptible to distortion and fading. The rapid changes in signal strength and phase caused by auroral propagation can make SSB signals difficult to understand.

3. **FM (Frequency Modulation):**
   - FM is even more affected by the rapid fading and distortion of auroral propagation. FM signals require a relatively stable signal to maintain intelligibility, which is not typically available in auroral conditions.

4. **RTTY (Radio Teletype):**
   - RTTY is a digital mode that is also affected by the rapid fading and distortion. The phase and frequency shifts caused by auroral propagation can lead to errors in decoding RTTY signals.

### Summary:

- **CW** is preferred for auroral propagation due to its narrow bandwidth and resilience to distortion.
- **SSB, FM, and RTTY** are less effective because they are more susceptible to the rapid fading and distortion typical of auroral conditions.