Skip to main content

Ionosphere layers and characteristics

Short answer

The ionosphere comprises D (50–90 km, day-only, absorbs), E (90–140 km, reflects MF), F1 (150–250 km, day) and F2 (250–400 km, highest density, main HF reflector). Its refractive index n=√(1−81N/f²) bends waves; critical frequency fc=9√Nmax, MUF=fc secθi. It varies with sun and can cause fading and blackouts.

Explain the different layers of ionosphere. What are the major characteristics of ionosphere?

Layers of the ionosphere

The ionosphere is the region of the upper atmosphere, about 50–400 km high, ionised by solar UV and X-ray radiation. It is divided into layers by height and electron density:

 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
  • D layer: lowest; daytime only; high collision rate, so it absorbs MF and low-HF waves; reflects VLF/LF.
  • E layer: reflects MF and low HF; weak at night; sporadic-E patches can reflect VHF.
  • F1 layer: daytime only; merges into F2 at night; partly reflects and partly absorbs HF.
  • F2 layer: highest and densest; present day and night; main layer for long-distance HF; single hop up to ≈ 4000 km.

Major characteristics of the ionosphere

  1. Variable ionisation: depends on the sun, so it changes with time of day (diurnal), season, latitude and the 11-year sunspot cycle.
  2. Refractive index less than 1: n = √(1 − 81N/f²); decreases as electron density rises, so waves bend back to earth.
  3. Frequency dependent: higher frequencies penetrate further; above the critical frequency (fc = 9√Nmax) at vertical incidence, and above the MUF for oblique incidence, waves escape.
  4. Absorption: proportional to collision frequency and to 1/f², greatest in the D layer.
  5. Irregularities and disturbances: sporadic E, sudden ionospheric disturbances (solar flares) and ionospheric storms cause fading and blackouts.
  6. Virtual height is always more than actual height.
  7. Acts as a dispersive, time-varying, slightly anisotropic medium (because of earth's magnetic field, causing Faraday rotation).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Propagation and Antenna chapter 4 questions →