At what time of year is Sporadic E propagation most likely to occur?
A. Around the solstices, especially the summer solstice
B. Around the solstices, especially the winter solstice
C. Around the equinoxes, especially the spring equinox
D. Around the equinoxes, especially the fall equinox
Sporadic E Propagation refers to the phenomena where radio signals bounce off of pockets of ionized gas in the (wait for it) E layer of the ionosphere. The “sporadic” refers to the randomness inherent in a signal actually hitting one of these pockets and making the bounce. It mainly affects VHF-range signals, but can occur all the way down into 10m
This differs from the more regular and understood “F” layer propagation where signals are effectively “bent” by the higher altitude F layer.
What exactly causes this to happen? Nobody knows. Seriously I tried to find a source to explain this in detail, but the best I could come up with is “No conclusive theory has yet been formulated as to the origin of Sporadic E.” (Wikipedia)
Barring any decent way to explain the reasons, I guess we’d better file this question under “just memorize it.” And in that, the answer is A. Around the solstices, especially the summer solstice. This is when its observed the most.