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

*Correct Answer is  A: CW*  

### AI Response: (may contain errors)  

To determine which emission mode is best for auroral propagation, let's analyze the options:

### Correct Answer: A: CW (Continuous Wave)

1. **Why CW is Correct:**
   - **Propagation Characteristics:** CW is a simple, narrowband mode that can penetrate the ionized layers created by auroras. The signal is less affected by the scattering and absorption that occurs in auroral conditions.
   - **Signal Strength:** CW signals can be received even when they are weak, making them more effective for long-distance communication during auroral events.

### Why the Other Choices are Incorrect:

2. **B: SSB (Single Sideband)**
   - **Bandwidth:** SSB is wider than CW and requires more signal strength to be effective. During auroral conditions, the signal may be distorted or weakened, making it less reliable than CW.

3. **C: FM (Frequency Modulation)**
   - **Propagation Limitations:** FM is typically used for local communications and has a higher power requirement. It is less effective for long-distance propagation, especially through auroral disturbances.

4. **D: RTTY (Radio Teletype)**
   - **Data Rate and Bandwidth:** RTTY is a digital mode that requires a stable signal and bandwidth. The unpredictable nature of auroral propagation can lead to errors in data transmission, making it less suitable than CW.

### Summary:
- **Best Mode for Auroral Propagation:** CW is preferred due to its narrow bandwidth and ability to be decoded even at low signal levels.
- **Other Modes:** SSB, FM, and RTTY are less effective due to their wider bandwidths and higher power requirements, which are not ideal under auroral conditions. 

This understanding will help you choose the right mode for effective communication during auroral events.