Question E3A12:
What is most likely to result in auroral propagation?  

Choices:  
A: Meteor showers  
B: Quiet geomagnetic conditions  
C: Severe geomagnetic storms  
D: Extreme low-pressure areas in polar regions    

Correct Answer is  C: Severe geomagnetic storms  

Correct answer: Severe geomagnetic storms (Choice C)

Why C is correct:
- Severe geomagnetic storms inject energetic particles into the high-latitude upper atmosphere. These create intense, patchy ionization and field-aligned irregularities in the E region (about 90–120 km).  
- Those irregularities cause auroral backscatter, enabling VHF contacts (most commonly on 50 and 144 MHz) over paths roughly 500–2000 km, with characteristic raspy, fluttery signals.

Why the other choices are incorrect:
- A: Meteor showers produce brief ionized trails that support meteor scatter, not auroral backscatter. Different mechanism and timing.
- B: Quiet geomagnetic conditions mean little or no aurora; insufficient irregularities for auroral propagation.
- D: Extreme low-pressure areas are tropospheric weather. Aurora is an ionospheric/magnetospheric phenomenon, not driven by surface weather.

Useful formulas and rules of thumb:
- Look for high geomagnetic indices:  
  - Kp index: auroral propagation becomes likely at about $Kp \ge 5$ and strong at $Kp \ge 7$.  
  - Large A index also indicates disturbed conditions.
- Plasma frequency (context for ionization level):  
  $$f_p \approx 9\sqrt{N_e}\ \text{kHz}$$  
  where $N_e$ is electron density in $\text{electrons/cm}^3$. Auroral events raise $N_e$ in the E region, enhancing scatter (though auroral propagation is primarily from irregularities, not simple specular reflection).
- Operating tips:  
  - Aim your beam toward the auroral oval (generally north in the Northern Hemisphere) for backscatter.  
  - Expect strongest effects on 50 and 144 MHz; sometimes 222/432 MHz during major storms.  
  - Signals are distorted with Doppler spread; CW and slower-speed modes copy better.  
  - HF often suffers absorption during severe storms—auroral propagation is mainly a VHF phenomenon.