The Ionospheric Mirror

Plate 76 speaks for itself: “Below about 9 MHz on a typical day, the sky is a mirror.”

The Physics

Radio waves below the plasma frequency of the ionosphere's F2 layer cannot propagate in it and are reflected back. Appleton and Barnett proved the layer exists in 1924.

f_p ≈ 8.98 √n_e Hz

• f_p: plasma frequency, Hz
• n_e: electron density, m⁻³

F2 layer, n_e ~ 10¹² m⁻³, gives ≈ 9 MHz. Appleton & Barnett showed it, 11 Dec 1924.

Honest caveat: The critical frequency ranges from about 2 to 15 MHz with time of day, season and solar cycle, and applies to waves sent straight up; oblique waves reflect at higher frequencies.

Sources

• E. V. Appleton, M. A. F. Barnett, Nature 115, 333 (1925).
• K. Davies, Ionospheric Radio (1990).

On the poster

Plate 076, “The Ionospheric Mirror”: the pixel-art plate with its slogan “Below about 9 MHz on a typical day, the sky is a mirror.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.