1. The innermost layer of the ionosphere. It occurs at heights of 50 to 90 km during daylight hours, reflects ELF, VLF, and LF waves, absorbs MF waves, and partially absorbs HF waves. Some consider the D-layer as starting at 70-80 km and merging with the E-layer. See Figure A-22. 2. A layer within the Earth; see Figure E-1.