F1layer

F1layer

[¦ef ′wən ‚lā·ər]
(geophysics)
The ionosphere layer beneath the F2 layer during the day, at a virtual height of 120-180 miles (200-300 kilometers), being closest to earth around noon; characterized by a distinct maximum of free-electron density, except at high latitudes during winter, when the layer is not detectable.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.