The Estimation of D-Region Electron Densities From Trans-Ionospheric Very Low Frequency Signals

Due to the relatively low ionization levels present in the D-region of the ionosphere (60-90 km), few reliable methods exist for estimating electron densities at this altitude. Very Low Frequency (VLF, 3-30 kHz) radio waves, unlike higher frequency waves used for similar remote sensing applications, are sensitive to these low ionization levels. Much past work has addressed the estimation of D-region electron densities using ground-based measurements of signals originating from VLF transmitters or atmospheric lightning. We introduce a new technique for the estimation of ionospheric D-region electron densities using satellite observations of trans-ionospheric VLF signals originating from high-powered VLF transmitters. Although most of the energy radiated from these transmitters is confined to sub-ionospheric propagation modes, a small fraction penetrates the ionosphere in a right-hand circularly polarized "whistler mode" which has been observed by many satellites. We show that the modal interference pattern previously observed enables the estimation of the electron density profile in the D-region from a single satellite pass. Using data from the DEMETER experiment, each collection is compared with results from a validated theoretical model to select a best-fit D-region electron density profile. Results show expected seasonal variations and are consistent with rocket observations. Although the D-region electron density estimates derived with this technique are shown to exhibit long-term seasonal variations consistent with rocket and ground-based observations, it is important to note that validation of these results remains difficult due to the lack of other techniques for continuously sensing the D-region.