A daytime region of the ionosphere ranging in height from approximately 30-50 miles. Radio wave absorption in this region can significantly increase in response to increased ionization associated with changes in solar electromagnetic emissions, e.g., x-ray flares. Under normal conditions, D region absorption drops off dramatically above about 9 MHz but, following an x-ray flare, signals up to and beyond 30 MHz can be significantly degraded. The D region is the lowest region of the ionosphere.