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Why can increased ionospheric density interfere with shortwave radio signals?

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Space weather Earth's full North Polar auroral oval, in an image taken in ultraviolet light by the U.S. Polar spacecraft over northern Canada, April 6, 1996. In the colour-coded image, which simultaneously shows dayside and nightside auroral activity, the most intense levels of activity are red, and the lowest levels are blue. Polar, launched in February 1996, was designed to further scientists' understanding of how plasma energy contained in the solar wind interacts with Earth's magnetosphere.

Increased ionospheric density can interfere with shortwave radio signals because of how the ionosphere interacts with electromagnetic waves. While the ionosphere normally acts as a mirror that reflects radio signals back to Earth, allowing for long-distance communication, excessive ionization—often triggered by increased solar activity—changes this dynamic.123

When solar activity increases, the density of the ionospheric layers rises. While this can initially improve reflectivity, further increases in density lead to the absorption of radio waves rather than their reflection. Specifically, intense solar radiation can ionize the lowest layer of the ionosphere, known as the D layer. Unlike the higher F layers, the D layer is non-reflecting at shortwave frequencies and instead acts to attenuate or absorb the energy of the propagating radio wave.214

Consequently, when ionospheric density becomes too high, the signals that would normally skip off the upper layers are instead absorbed by the lower, denser regions, effectively hampering or causing a complete fade-out of shortwave communications.24