A Modern VLF Receiver for use in an Array for VLF Imaging of the D-Region

Very Low Frequency (VLF) radio receivers have been used to remotely sense the ionosphere and whistler waves traveling through it for over 60 years (R.A. Helliwell, Stanford University Press, Stanford (1965)). A common technique to remotely sense ionospheric density uses the measurement of narrowband signals from different VLF transmitters around the world. This technique relies on the VLF signals propagating efficiently within the Earth-Ionosphere waveguide and the ability of changes in measured amplitude and phase to be tied to changes in the D-region ionospheric density profile. As a path-integrated measurement, a single receiver can measure average changes to the D-region density along the transmit-receive path but cannot be used to probe small spatial scale disturbances along that path.