Earth–ionosphere waveguide Earth and the ionosphere. The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7 Hz. 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 (f) of the F-layer maximum (1) f = 9 (N) kHz (N in cm is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than f are reflected within the ionospheric D-, E-, and F-layers. f is of the order of 8–15
Atmospheric diffraction Earth's surface. This phenomenon is sometimes called the corona effect, not to be confused with the solar corona. On the right is a 1/10-second exposure showing an overexposed full moon. The Moon is seen through thin vaporous clouds, which glow with a bright disk surrounded by an illuminated red ring. A longer exposure would show more faint colors beyond the outside red ring. Another form of atmospheric diffraction or bending of light occurs when light moves through fine layers of particulate dust trapped primarily in the middle layers of the troposphere. This effect differs from water based atmospheric diffraction because