Advances in Near-transmitter VLF Remote Sensing: Auroral Precipitation Effects on the D-region Ionosphere

Abstract:

VLF transmitters are operated by the U.S. Navy for long-distance communications. Close to the transmitter, ground-wave and sky-wave propagation, both of which can be strongly dispersive, significantly affect the received signal. A newly developed time-of-arrival analysis technique, which accounts for ground-wave and skywave propagation, is employed to infer the effective reflection height of VLF radio waves measured at a receiver close to (~20 km from) the Cutler, Maine VLF transmitter (NAA). In this paper, the improved TOA method is described and applied to observations of strong auroral precipitation. Although the auroral oval is typically northward of NAA, it can dip equatorward during periods of high geomagnetic activity. During such an event, energetic electrons can penetrate deeply into the upper atmosphere, enhancing the electron density of the D-region ionosphere through the production of secondary ionization. For the cases analyzed, the TOA-determined changes to the effective reflection height during several auroral precipitation events are interpreted in the context of an electron precipitation and ionospheric deposition model.