F layer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The highest region of the ionosphere, extending at night from about 190 to 400 kilometers (120 to 250 miles) and during the day from about 145 to 400 kilometers (90 to 250 miles) above the earth.
• noun Either of two layers, designated F₁ and F₂, into which this region is divided during the day, extending respectively from about 145 to 240 kilometers (90 to 150 miles) and from about 190 to 400 kilometers (120 to 250 miles) above the earth.