DEMETER Observations of Ionospheric Heating Time Constants Above the NWC VLF Transmitter

As demonstrated by recent DEMETER observations, intense 19.8 kHz VLF signals from the powerful (1 MW) NWC transmitter in Australia significantly heat the overlying ionosphere and produce significant changes in local electron and ion density and temperature at 700 km altitude [Parrot et al., 2007]. These changes are accompanied by a unique VLF plasma wave structure covering a 5 to 10 kHz band below the NWC signals and by quasi-electrostatic ELF turbulence. In order to determine the heating and cooling time constants of this effect, a campaign was carried out in which the NWC transmitter was programed to transmit a single CW pulse of 2 second duration every 10 seconds during periods in which the DEMETER spacecraft passed over the transmitter location. The data from this campaign show that the time constant for production of the unique VLF plasma wave structure and the quasi-electrostatic ELF turbulence ranged from 100 to 300 msec. However significant changes in electron and ion density and temperature during the 2 second pulses occurred only sporadically. This result suggests that small scale ( 10-100 m) plasma density irregularities are produced quickly by the heating pulses, but larger scale irregularities take significantly longer than 2 seconds to develop. We discuss the observations obtained during the campaign and the physical mechanisms involved in the heating process.