DEMETER Observations of Ionospheric Heating by Powerful VLF Transmitters

Recent DEMETER observations have shown that intense 19.8 kHz VLF signals from the powerful (1 MW) NWC transmitter in Australia can significantly heat the overlying ionosphere and produce significant changes in local electron density and temperature at 700 km altitude [Parrot et al., 2007]. These changes are accompanied by a unique plasma wave structure covering a 5 to 10 kHz band below the NWC signals. We extend the previous investigation to the powerful (1 MW) 24 kHz NAA VLF transmitter in Maine and show that similar heating effects take place in the ionosphere overlying NAA. However, the heating due to the NAA signals occurs at a much lower level than that due to the NWC signals. We show that this important difference arises because the ionospheric absorption rate for VLF waves depends upon the local inclination of the Earth's magnetic field. The magnetic inclination varies significantly between the locations of NWC and NAA, being approximately 45 and 67 degrees respectively. We also examine the plasma wave structure and the electron density and temperature near the conjugate point of NWC and show that although the plasma wave structure is very similar to the unique structure found over NWC, there are no significant perturbations of electron density or temperature near the conjugate point . This implies that the large scale changes in electron density observed over NWC do not extend along the Earth's magnetic field lines to the conjugate point.