The ionosphere is a part of Earth’s upper atmosphere that is ionized by solar radiation, meaning it contains a high concentration of free electrons and ions, concentrated in regions, as illustrated below:
 A model of the ionospheric regions
This figure depicts ionospheric regions, labeled D, E, F1, and F2, with an additional label F on the night side. It highlights the variation in ionosphere layers between day and night, with the day side on the left and the night side on the right. This visual representation helps understand how the ionosphere's structure changes with the time of day.
These ionospheric regions consist of "clouds" or "bubbles" of free electrons, as illustrated below:
The moving plasma clouds are travelling disturbances of electron density
The dynamic ionosphere causes signal fading (QSB) over time. Small-scale irregularities in the ionosphere are observed at all levels, with periodic motions attributed to neutral atmospheric waves interacting with ionized components in the upper atmosphere. While understanding is limited, the research promises short-term changes.
Read a Summary of the theoretical topics of HF radio propagation. See also an index of terms for HF Radio Propagarion.
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