Ionosphere layers and their importance

The ionosphere is the ionised part of the upper atmosphere, from about 50 km to 400 km above the earth. Solar UV and X-rays knock electrons off gas molecules; these free electrons refract radio waves. The ionosphere has several layers with different heights and electron densities.

 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 and their importance

• D layer (50–90 km): lowest and weakest; exists only in daytime and vanishes after sunset. Its high collision rate makes it an absorbing layer: it absorbs MF and lower HF sky waves (absorption ∝ 1/f²). This is why AM (MW) stations are heard only by ground wave in daytime. It reflects VLF/LF waves (earth–ionosphere waveguide, used for navigation).
• E layer (90–140 km): mostly a daytime layer, weak at night. Reflects MF and lower HF waves; gives single-hop ranges up to about 2000 km. At night the residual E layer gives long-distance MW reception. Sporadic E patches can reflect even VHF signals irregularly, causing long-distance TV/FM interference.
• F1 layer (150–250 km): present only in daytime; merges with F2 at night. Reflects some HF waves but mainly adds absorption to waves going to F2.
• F2 layer (250–400 km): highest and most strongly ionised; present day and night. It is the most important layer for long-distance HF (SW) communication: highest critical frequency (up to ≈ 12 MHz), single hop up to ≈ 4000 km, and round-the-world links by multi-hop. Its height and density vary greatly with season and sunspot cycle.

Overall importance to radio communication

1. Makes long-distance HF communication and SW broadcasting possible with modest power, without satellites.
2. Decides critical frequency, MUF and skip distance for each path and time.
3. Causes absorption (D layer) and fading, so frequency must be chosen by time of day (higher by day, lower at night).
4. Waves above ≈ 30 MHz pass through, which allows satellite and space communication at VHF and above.