The region of the earth's upper atmosphere containing a small
percentage of free electrons and ions produced by photoioniza-
tion of the constituents of the atmosphere by solar ultraviolet
radiation at very short wavelengths (l.t.1000 angstroms). The
ionosphere significantly influences radiowave propagation of fre-
quencies less than about 30 MHz.
A disturbance in the F REGION of the IONOSPHERE, which
occurs in connection with geomagnetic activity.