Low-frequency waves in HF heating of the ionosphere

Ionospheric heating experiments have enabled an exploration of the ionosphere as a large-scale natural laboratory for the study of many plasma processes. These experiments inject high-frequency (HF) radio waves using high-power transmitters and an array of ground- and space-based diagnostics. This chapter discusses the excitation and propagation of low-frequency waves in HF heating of the ionosphere. The theoretical aspects and the associated models and simulations, and the results from experiments, mostly from the HAARP facility, are presented together to provide a comprehensive interpretation of the relevant plasma processes. The chapter presents the plasma model of the ionosphere for describing the physical processes during HF heating, the numerical code, and the simulations of the excitation of low-frequency waves by HF heating. It then gives the simulations of the high-latitude ionosphere and mid-latitude ionosphere. The chapter also briefly discusses the role of kinetic processes associated with wave generation

DEMETER observations of an intense upgoing column of ELF/VLF radiation excited by the HAARP HF heater

International audienceDEMETER spacecraft observations of ELF signals generated by the recently upgraded High-Frequency Active Auroral Research Program (HAARP) HF facility (3.6 MW) reveal three distinctive regions characterizing upgoing ELF waves. These regions are classified by signal intensity and the minimum lateral distance d between the magnetic footprint of the satellite at 75-km altitude (D layer) and the point at 75-km altitude immediately above HAARP where the source is located. The first large region within d ' 900 km contains waves which propagate in the Earth-ionosphere waveguide and then leak upward to the spacecraft. The second region of d ' 200-300 km contains waves propagating to the spacecraft from the ionospheric source region without reflection from the ground. The third region contains waves of very high intensity (E ' 350 mV/m, B ' 20 pT) within a narrow cylindrical column of $10-20 km radius, also observed once before on the ISIS 1 spacecraft. The observed intense columnar radiation is consistent with predictions of a recent full-wave model of ELF radiation from HF-heater-produced ionospheric source currents

Effects of plasma density irregularities on the pitch angle scattering of radiation belt electrons by signals from ground based VLF transmitters

International audienceRecent DEMETER spacecraft observations show that VLF signals from the NPM transmitter in Hawaii often strongly excite quasi-electrostatic whistler mode waves as the NPM signals propagate upward through plasma density irregularities. As a result of the NPM wave energy loss to the quasi-electrostatic waves, the transmitter signals will arrive at the radiation belts with less intensity than predicted by present models of VLF wave propagation and will produce less pitch angle scattering of energetic electrons than presently believed. This type of wave energy loss may be partially responsible for the pervasive wave intensity deficit for VLF transmitter signals in the plasmasphere recently noted by Starks et al. (2008)

DEMETER observations of ionospheric heating by powerful VLF transmitters

International audience[1] We report DEMETER spacecraft observations of iono-spheric heating produced above powerful VLF transmitters by their intense radiated electromagnetic (EM) signals. We compare the heating effects of signals from the 1 MW NWC transmitter in Australia with those produced by signals from the 885 kW NAA transmitter in Maine. Significant observable effects include perturbations in plasma density and thermal electron temperature, and the production of quasi‐electrostatic (QE) VLF plasma wave bands, both over the transmitters, and, in the case of NWC, also in the magnetically conjugate region. In the regions in which the QE wave bands were observed, they were invariably accompanied by a band of ELF turbulence with maximum intensity below 300 Hz. Such turbulence has in the past been associated with the presence of small scale plasma density irregularities. This association suggests that heating effects due to NWC are far‐reaching and extend along B o into the conjugate hemisphere where they are expressed in part as small scale plasma density fluctuations

DEMETER satellite observations of lightning-induced electron precipitation

International audienceDEMETER spacecraft detects short bursts of lightninginduced electron precipitation (LEP) simultaneously with newly-injected upgoing whistlers, and sometimes also with once-reflected (from conjugate hemisphere) whistlers. For the first time causative lightning discharges are definitively geo-located for some LEP bursts aboard a satellite. The LEP bursts occur within <1 s of the causative lightning and consist of 100-300 keV electrons. First in-situ observations of large regions of enhanced background precipitation are presented. The regions are apparently produced and maintained by high rate of lightning within a localized thunderstorm

Modeling of Doppler-shifted terrestrial VLF transmitter signals observed by DEMETER

International audienceObservations of signals from a terrestrial very low frequency (VLF) transmitter made by the DEMETER spacecraft inside the plasmasphere are modeled using a three-dimensional wave propagation code. The simulation results agree well with the satellite measurements, predicting both the incidence and frequency offset of Doppler-shifted signals resulting from non-ducted interhemispheric propagation paths through the plasmasphere. The observed Doppler shifts are similar to those which can result from linear mode coupling as VLF transmitter signals scatter from small-scale plasma density irregularities. Thus care must be taken to differentiate the two effects when studying the power loss of VLF waves through the ionosphere. The agreement shown between predictions and observation demonstrates the utility of the models used for understanding the wave energy distribution in the plasmasphere from terrestrial transmitters

DEMETER observations of transmitter-induced precipitation of inner radiation belt electrons

International audienceNear loss cone energetic electron flux increases induced by ground-based very low frequency (VLF) transmissions are observed directly via satellite-based detection. In 2 years of experiments ranging from 27 March 2006 through 2 April 2008 with the 21.4-kHz transmitter NPM in Lualualei, Hawaii, and the French satellite DEMETER (detection of electromagnetic emissions transmitted from earthquake regions), only a few cases of detection of individual pulses of transmitter-induced precipitation of inner radiation belt electrons have been realized. Analysis of the specific cases of detection allow comparison of precipitating flux with predictions based on ray-tracing analyses of wave propagation and test particle modeling of the wave-particle interaction. Results indicate that the precipitated flux of >100 keV electrons induced by the NPM transmitter peaks at L ' 1.9 and, in the rare cases of detection, may be at higher energies than the $100 keV peak predicted by the model. The low detection rate is attributed to the orientation of the DEMETER particle detector, which is mostly overwhelmed by the trapped population at the location of detection