Answer: Electron density in the ionosphere (f c = 9√N where N = electron density).
- A Electron density in the ionosphere (f<sub>c</sub> = 9√N where N = electron density)
- B The absolute temperature of the ionospheric layer alone
- C Earth's magnetic field strength alone, with no electron density term
- D The physical height of the ionospheric layer alone
Correct answer: A. Electron density in the ionosphere (f<sub>c</sub> = 9√N where N = electron density)
Explanation: Plasma frequency f<sub>p</sub> = (1/2π)√(Ne²/ε_0 m<sub>e</sub>) = 9√N (in SI). Waves above this frequency pass through the ionosphere; waves below are reflected. This determines which frequencies can be used for long-range HF communications.
In an electromagnetic wave, the oscillating electric field (E) and magnetic field (B) are perpendicular to each other and to the direction the wave travels - a purely transverse wave that needs no medium, unlike sound.
Concept context
Maxwell's equations, EM spectrum, properties, and applications of each band.