General: Ionosphere’s Maximum Height

The General question of the week is from sub-element 3 (propagation) section C (ionospheric layers) [G3C02]

Where on the Earth do ionospheric layers reach their maximum height?

A. Where the Sun is overhead
B. Where the Sun is on the opposite side of the Earth
C. Where the Sun is rising
D. Where the Sun has just set

The answer to this question makes the most sense if you look at the following image:

Ionospheric absorption

As you can see, the ions in the ionosphere get there because of the sun.  The sun emits charged particles, aka the solar wind.  The part of the earth that catches most of these particles, is of course, the part directly facing the sun.  The charged radiation collects here, and excites the atmosphere, so the top layers increase in height.  Where the sun is setting or rising, the angle these particles hit the earth is so low, they reflect right off.  And of course, the part facing away from the sun, doesn’t get any exposure at all.

Therefore, the answer is A. Where the Sun is overhead. 

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