text: Chapter 6
  Figure 6-8. Structure of ionosphere.
  6-82. The majority of HF sky-wave communications depend on the F1 and F2 regions. The F2 region is
  used the most for long-range daytime communications.
  6-83. The E region is the next lower region. It is present 24 hours a day, although at night it is much
  weaker. The E region is the first region with enough charge to bend radio signals. At times, parts of the
  E-region become highly charged. This can either help or block HF communications. These highly charged
  areas are called "sporadic E." They occur most often during the summer.
  6-84. The D-region is closest to earth and only exists during the day. It cannot bend a radio signal back
  to earth, but it does play an important role in HF communication. The D-region absorbs energy from the
  radio signal passing through it, thereby reducing the strength of the signal.
  6-85. The bending of the radio signal by the ionosphere depends on the frequency of the radio signal,
  the degree of ionization in the ionosphere, and the angle at which the radio signal strikes the ionosphere. At
  a vertical (straight up) angle, the highest frequency bent back to earth is called the critical frequency. Each
  region of the ionosphere (E, F1, and F2) has a separate critical frequency. For a vertical angle, signals
  above the highest critical frequency pass through all ionospheric regions and into outer space. Frequencies
  below the critical frequency of a region are bent back to the earth by that region; however, if the frequency
  is too low, the signal is absorbed by the D region. To have HF sky-wave communication, a radio signal
  must be a high enough frequency to pass through the D region, but not so high a frequency that it passes