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What is ionosphere; its propagation

Short answer

The ionosphere (50–400 km) contains D, E, F1, F2 layers ionised by solar radiation. Sky-wave propagation refracts HF waves back; D absorbs MF, E reflects MF/low HF, F2 reflects HF for long distances. Refraction occurs because n = √(1−81N/f²) decreases with height; critical frequency fc = 9√Nmax, MUF = fc sec θ.

What is ionosphere? Explain the ionosphere wave propagation showing its different layers.

Ionosphere

The ionosphere is the upper region of the atmosphere, about 50 km to 400 km above the earth, where solar ultraviolet and X-ray radiation ionises the gas, producing free electrons and ions that can bend radio waves back to earth.

Ionospheric wave propagation

In ionospheric (sky-wave) propagation, a wave (mainly HF, 3–30 MHz) is sent upward, refracted back by an ionised layer and received hundreds to thousands of km away.

 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

Layers

  • D (50–90 km): daytime only; absorbs MF/low HF; reflects VLF/LF.
  • E (90–140 km): reflects MF and lower HF; weak at night; sporadic-E patches.
  • F1 (150–250 km): daytime only; merges with F2 at night.
  • F2 (250–400 km): strongest, day and night; main reflector for long-distance HF.

Mechanism

  1. Refractive index n = √(1 − 81N/f²) falls as electron density N rises with height, so the ray bends away from the normal and turns back.
  2. Critical frequency fc = 9√Nmax is the highest frequency returned at vertical incidence; for oblique incidence the limit is MUF = fc sec θi.
  3. Waves above the MUF for their angle of incidence escape into space. For f > fc, steep rays pass through the layer, so no sky wave lands inside the skip distance (the skip zone).
  4. Repeated reflections between earth and ionosphere (multi-hop) give worldwide range.
  5. The ionosphere changes with time of day, season and sunspots, so frequencies are changed accordingly and signals may fade.

✓ Answer checked by Claude Opus 5.5 (high effort)

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