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Ionospheric wave propagation and layers

Short answer

The ionosphere (50–400 km) contains D, E, F1, F2 layers with varying electron density. Sky waves reflect by refraction; the highest reflected frequency is MUF = fc sec θi, with fc = 9√Nmax. Propagation enables long-distance HF communication via multiple hops, strongest at night when D layer disappears.

Explain the ionospheric wave propagation (operation of ionospheric communication) showing its different layers, with the structure of the ionosphere.

Ionospheric (sky-wave) propagation is the mode in which radio waves (mainly HF, 3–30 MHz) sent upward from the earth are bent back (refracted) by the ionised layers of the upper atmosphere and return to earth far away, giving long-distance communication of thousands of km.

Structure of the ionosphere

The ionosphere is the region from about 50 km to 400 km height where ultraviolet and X-ray radiation from the sun ionises gas molecules, producing free electrons and ions. Ionisation depends on sunlight, so it varies with time of day, season, latitude and the 11-year sunspot cycle. It is divided into layers:

 height (km)
  400 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ F2 (250-400)
  300 |        /\                    day & night
  220 |~~~~~~~/~~\~~~~~~~~~~~~~~~~~~ F1 (150-250)
      |      /    \                  day only
  110 |~~~~~/~~~~~~\~~~~~~~~~~~~~~~~ E  (90-140)
   70 |::::/::::::::\::::::::::::::: D  (50-90)
      |   /  sky     \               day only
    0 |__/____________\_____________ earth
       Tx    skip      Rx
LayerHeightElectron density (per m³)PresentMain effect
D50–90 km10⁸–10¹⁰Day onlyAbsorbs LF/MF sky wave; reflects VLF
E90–140 km (≈110)≈10¹¹Mostly day (weak at night)Reflects MF and lower HF; fc ≈ 3–4 MHz
Sporadic Eₛ≈110 kmPatchy, highIrregularOccasional VHF long-distance
F1150–250 km (≈200)≈2–5 × 10¹¹Day onlySome HF reflection, absorption; fc ≈ 4–5 MHz
F2250–400 km≈10¹² (highest)Day and nightMain HF reflector; fc ≈ 5–12 MHz; longest hops

At night the D layer disappears, F1 merges with F2 and E becomes weak, so night-time sky-wave propagation uses mainly F2 (and the residual E layer).

Operation of ionospheric communication

  1. Refraction, not mirror reflection: the refractive index of an ionised layer is n = √(1 − 81N/f²) (N in electrons/m³, f in Hz). As the wave goes up, N increases, n falls below 1, and the ray bends away from the normal, gradually turning back down.
  2. Condition to return: the wave returns if at some height sin θi = n, i.e. f ≤ 9√Nmax sec θi. Higher frequencies need oblique incidence; the highest frequency that returns for a given path is the MUF = fc sec θi.
  3. Critical frequency fc = 9√Nmax: the highest frequency reflected at vertical incidence.
  4. Skip distance and skip zone: for f > fc, waves at steep angles escape; the nearest point where the sky wave returns is the skip distance. Between the end of the ground wave and this point no signal is received (skip zone).
  5. Multi-hop: the returned wave reflects from earth and goes up again, so signals travel round the world in several hops (one F2 hop ≈ up to 4000 km).
  6. Virtual height: the height from which a mirror reflection would give the same delay; used in calculations.
  7. Absorption is largest in the D layer and at low frequency (∝ 1/f²), which is why MF sky wave is weak in daytime.
  8. Frequency choice: the optimum working frequency (OWF ≈ 0.85 MUF) is used; frequency is changed between day (higher) and night (lower).

Merits: very long range with low power; cheap. Demerits: fading, skip zone, dependence on sun and time, ionospheric storms, limited bandwidth.

✓ Answer checked by Claude Opus 5.5 (high effort)

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