Ionospheric D layer (50-90 km altitude) is the region where the collision between electron density and the electron-neutral is frequent. When HF radio waves go into ionosphere, the energy of radio waves is converted into heat and electromagnetic (EM) noise through interactions with free electrons and the neutral atmosphere. The process is called absorption. The radio wave signal can suffer attenuation or blackout due to the absorption.
The solar flares X-ray emission can ionize the D region significantly, increasing local electron density, and hence the radio wave absorption. Due to geometric effects, D region ionization by solar X-rays is greatest at the sub-solar point, and almost unaffected at the Earth's night-side.
The D region absorption prediction product consists of three components: a global map of HF radio wave absorption due to solar X-ray flux, an attenuation bar graph, and an estimated recovery clock. The prediction result will update according to the 1-min X-ray flux measurements from GOES satellites.