Which ionospheric region is closest to the surface of Earth?

The ionosphere is divided into several layers based on altitude. From lowest to highest, these are alphabetically named: D, E, and F (which splits into F1 and F2 during the day). The D region is the closest to the Earth's surface, typically found at an altitude of about 30 to 60 miles (50 to 90 km). Because it is the lowest and densest layer, it is primarily responsible for absorbing lower-frequency HF radio waves during daylight hours. The E and F regions sit higher up in the atmosphere and are primarily responsible for refracting (bending) signals back to Earth.