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How do solar flares cause radio fade-outs?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

Solar flares cause radio fade-outs through the emission of high-energy radiation that alters the Earth's ionosphere.

When a solar flare occurs, it releases intense pulses of "hard" X-rays—those with shorter wavelengths—which travel to Earth and penetrate deep into the atmosphere. These X-rays reach the lowest region of the ionosphere, known as the D-layer.123

The absorption of this high-energy radiation significantly increases the ionization density within the D-layer. While the ionosphere typically reflects radio waves to facilitate long-distance communication, this increased density causes the radio waves to be absorbed by the atmosphere instead of being reflected. This process effectively hampers shortwave communications, resulting in the phenomenon known as a radio fade-out.1