Question E3B11:
At what time of day is sporadic-E propagation most likely to occur?  

Choices:  
A: Between midnight and sunrise  
B: Between sunset and midnight  
C: Between sunset and sunrise  
D: Between sunrise and sunset    

Correct Answer is  D: Between sunrise and sunset  

At what time of day is sporadic-E propagation most likely to occur?

Correct Answer: D — Between sunrise and sunset

1) Why the correct answer is correct
- Sporadic-E (Es) forms as dense patches (“clouds”) of ionization in the E layer (about 90–130 km).  
- These patches are most common during local daytime due to stronger solar-driven atmospheric tides and wind shears that converge metallic ions, plus higher daytime ionization.  
- Typical daily peaks: late morning and late afternoon/early evening, especially in late spring–summer (secondary peak in mid-winter).

2) Why the other choices are incorrect
- A: Between midnight and sunrise — Es can occur at night but is much less frequent; not the most likely period.  
- B: Between sunset and midnight — Some evening events happen, but overall probability is lower than during the day.  
- C: Between sunset and sunrise — Nighttime in general is least likely for Es formation.

3) Useful formulas and rules of thumb
- MUF via secant law for a given ionized layer:
  $$MUF \approx f_{oE} \sec \theta = \frac{f_{oE}}{\cos \theta}$$
  where $f_{oE}$ is the E-layer critical frequency and $\theta$ is the wave’s angle of incidence from vertical. Higher daytime $f_{oE}$ raises MUF and Es likelihood.
- Typical single-hop Es distance: about 1000–2300 km (600–1400 miles).
- Practical rule: Expect Es most in daylight, especially late morning and late afternoon, peaking in late spring–summer on 10 m and 6 m (and occasionally higher VHF).