VLF/LF pulse reflections from layers below the ionospheric D-region

Long wave ionospheric reflectivity data are described from experiments using a VLF/LF pulse ionosounding system. The sounding technique utilized pulses so short, that even at distances of a few hundred kilometers from the transmitter, ground wave and ionospherically reflected sky wave pulses were received separated in time. Pulse reflection data are shown that strongly suggest the simultaneous presence of at least two discrete reflections from different heights in the daytime ionosphere. The upper reflections are identified with the classical D-region, caused primarily by Lyman alpha radiation, while the low-altitude reflections, which occur from shortly before sunrise to shortly after sunset, are believed to be due to ionization caused by cosmic rays and photodetachment. The variability of the low altitude daytime ionosphere is described from VLF/LF pulse reflection data obtained during different seasons, at mid- and low-geomagnetic latitudes, and over different phases of the solar cycle. The effects of the low altitude ionization on the propagation of long radio waves are estimated.