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Why can a solar flare cause a shortwave radio blackout?

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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.

A solar flare causes a shortwave radio blackout primarily by altering the Earth's ionosphere, the region of the atmosphere that reflects radio waves to enable long-distance communication.123

When a solar flare occurs, it emits intense pulses of hard (short-wavelength) X-rays that travel to Earth at the speed of light. These X-rays penetrate deep into the atmosphere, where they ionize the lowest layer of the ionosphere, known as the D-layer.4123

Under normal conditions, the D-layer acts primarily to attenuate radio waves, while higher layers (the E and F layers) reflect them. However, the sudden increase in ionization caused by the flare's X-rays significantly increases the density of the D-layer. This heightened density causes the layer to absorb, rather than allow the passage of, the radio waves that would otherwise be reflected by the upper layers. Consequently, the radio signals are dissipated as heat within the D-layer, resulting in a temporary blackout of shortwave communications.231