the lowest area of the ionosphere, having increased ion density and existing only in the daytime: it begins at an altitude of about 70 km (c. 43 mi) and merges with the E layer
See D layer in American Heritage Dictionary 4
noun
The lowest region of the ionosphere, existing only during the day and extending from about 40 to 65 kilometers (25 to 40 miles) above the earth. Also called D region.