n.the highest region of the ionosphere (from 90 to 600 miles up) which contains the highest concentration of free electrons and is most useful for long-range radio transmission
同义词：F layerF region
英汉双解大词典
Appleton layer
1.
N（名词）
the F region of the ionosphere 阿普顿层; 大气最上面的F电离层