page_title: Earth–ionosphere waveguide
section_title: Introduction
hierarchical_section_title: Earth–ionosphere waveguide / Introduction
caption_attribution_description: English: Fieldstrength vs Distance of VLF wave (diagram)
context_page_description: The Earth–ionosphere waveguide refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere.
  Because the ionosphere contains charged particles, it can behave as a conductor. The earth operates as a ground plane, and the resulting cavity behaves as a large waveguide.
  Extremely low frequency and very low frequency
  signals can propagate efficiently in this waveguide. For instance, lightning strikes launch a signal called radio atmospherics, which can travel many thousands of kilometers, because they are confined between the Earth and the ionosphere.
  The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7 Hz.
context_section_description: Radio propagation within the ionosphere  depends on frequency, angle of incidence, time of day, season, Earth's magnetic field, and solar activity. At  vertical incidence, waves with frequencies larger than the  electron plasma frequency ( in Hz)
  of the F-layer maximum
  ( in  is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than  are reflected within the ionospheric D-, E-, and F-layers. fₑ  is of the order of 8–15 MHz during day time conditions. For oblique incidence, the critical frequency becomes larger.
  Very low frequencies (VLF: 3–30 kHz), and extremely low frequencies (ELF: <3  kHz) are reflected at the ionospheric D- and lower E-layer. An exception is whistler propagation of lightning signals along the geomagnetic field 
  lines.
  The wavelengths of  VLF waves (10–100 km) are already comparable with the height of the ionospheric D-layer (about 70 km during the day, and 90  km during the night). Therefore, ray theory is only  applicable for propagation over short distances, while mode theory must be used for larger distances. The region between Earth's surface and the ionospheric D-layer behaves thus like a waveguide for VLF- and ELF-waves.
  In the presence of the ionospheric plasma and the geomagnetic field, electromagnetic waves exist for frequencies which are larger than the gyrofrequency of the ions (about 1 Hz).  Waves with frequencies smaller than the gyrofrequency are called hydromagnetic waves. The geomagnetic pulsations with periods of seconds to minutes as well as Alfvén waves belong to that type of waves.
caption: Figure 3. Normalized vertical field strength Ez vs. distance ρ in magnitude (solid line, left ordinate) and phase (dashed line, right ordinate).[note 1]
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