Question E3B03:
At what time of day is transequatorial propagation most likely to occur?  

Choices:  
A: Morning  
B: Noon  
C: Afternoon or early evening  
D: Late at night    

Correct Answer is  C: Afternoon or early evening  

Transequatorial propagation (TEP) peaks when equatorial F‑region dynamics enhance the MUF along paths straddling the geomagnetic equator.

Why C (Afternoon or early evening) is correct
- Near local late afternoon into early evening (roughly 1500–2100 local time), the “prereversal enhancement” of the eastward electric field at the geomagnetic equator lifts and tilts F‑region plasma.  
- This strengthens the equatorial ionization anomaly and raises the MUF on paths symmetric about the magnetic equator, enabling TEP on 10 m and 6 m.
- Result: best TEP probability in the afternoon/early evening, especially near the equinoxes.

Why the others are incorrect
- A (Morning): The equatorial anomaly is not yet fully developed; MUF and irregularities needed for TEP are weaker.
- B (Noon): Although ionization is high, D‑layer absorption is also higher and the electric field geometry is less favorable; MUF along the TEP path is typically not at its peak.
- D (Late at night): After evening, the anomaly and irregularities decay, MUF drops, and TEP largely shuts down. Rare nocturnal TEP can occur, but it’s not the most likely time.

Useful rules of thumb
- Time window: Afternoon to early evening (about 1500–2100 local time).
- Seasons: Best near the spring and fall equinoxes.
- Bands: Most common on 28–50 MHz; evening events can extend higher.
- Geometry: Endpoints typically within about $10^\circ$–$20^\circ$ of the geomagnetic equator with the path midpoint near the magnetic equator; path lengths often 4000–8000 km.

Helpful relation
- MUF along a path is approximated by  
  $$MUF \approx f_oF2 \sec \theta$$  
  where $f_oF2$ is the F2 critical frequency and $\theta$ is the ray’s zenith angle. Around afternoon/evening, conditions near the magnetic equator raise $f_oF2$ and path MUF, favoring TEP.