Discussion
Critical frequency is \( f_{c} = 9\sqrt{N} \) with N in electrons per cubic metre. What is it for \( N = 10^{12} \)?
Answer: B
Taking the square root of the electron density and multiplying:
\[ f_{c} = 9 \times 10^{6} = 9 \times 10^{6} \]
hertz, that is 9 MHz. The square root dependence means a fourfold rise in electron density only doubles the critical frequency, which is why the usable band changes less dramatically through the solar cycle than the underlying ionisation does. Being able to move between electron density and critical frequency connects the physics to the operating practice directly.
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