Latitudinal cutoff of VLF signals in the ionosphere

The polar-orbiting OGO 2 satellite, receiving 18-kHz manmade VLF signals from the ground, shows a latitudinal cutoff of these signals between roughly 50° and 70° invariant latitude. The signal cutoff ranges from 10 to 40 db magnetic field attenuation and is frequently followed by strong noise believed to be auroral hiss. In about half of the cases the VLF signals from the ground again become detectable beyond the region of noise generation. There is a tendency for the cutoff to be more pronounced during the daytime than during nighttime, and the latitude of the cutoff tends to decrease with increasing Kp. Onset of the attenuation effect is sometimes as large as 40 db over a distance as small as a few kilometers. It is concluded that the latitudinal cutoff occurs over the ionospheric rather than the subionospheric portion of the path.