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International audience; [1] Bell et al. (2009) proposed that the source region for banded chorus consists of whistler mode ducts of depleted electron density (N e) for upper band (UB) chorus for all wave normal angles () and ducts of either enhanced or depleted N e for lower band (LB) chorus for small and near or greater than the Gendrin angle, respectively. This paper provides support for this model using new high resolution (17 km) N e observations from the Cluster WHISPER and EFW instruments. Data is examined from January 20, 2004, when strong banded chorus was observed over 3000 km of the Cluster 2 orbit, ending at the magnetic equator. Previous analysis of LB chorus on this day indicated the wave normal angles were larger than the Gendrin angle. Using the N e data, we show that the LB chorus is generated within depletion ducts in the source region and that the half‐width of these ducts (∼70 km) is comparable to the transverse scale (∼100 km) of the chorus source region. Making use of the fact that the group velocity of the LB chorus waves has a significant cross‐L component, we show that the source region extends over 500 km near the magnetic equator.

This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground‐based observations of chorus via a 3 month study of ground‐based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50°S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L ∼ 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L ∼ 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode.

International audience; The MF data recorded by the low-altitude satellite DEMETER have been used to survey the MF waves around the Earth. A global map of the MF emissions indicates that there exists a wave activity in the frequency range 2-2.5 MHz above the main powerful VLF ground-based transmitters operating in the frequency range 18-50 kHz. It is shown that this is due to the high-frequency part of whistlers induced by the thunderstorm activity. They can penetrate trough the ionosphere at the locations of the transmitters because these transmitters induce large ionospheric perturbations. This means that an integrated map over several months is able to show these MF emissions above all main VLF transmitters. The discrepancy between intensities of the emissions in winter and summer (in the Northern Hemisphere) is explained by considering the geographic variations of the plasma frequency below the satellite. It is shown that the MF waves spread in longitude in the hemisphere opposite to the VLF transmitters.

International audience; Near 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.

International audience; First recordings of satellite ELF/VLF waveform data associated with transient luminous event (TLE) observations are reported from the summer 2005 campaign coordinated by Stanford University and Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPCE). TLEs are optically observed from the U.S. Langmuir Laboratory, while ELF/VLF waveform data are simultaneously recorded on board the Centre National d'Etudes Spatiales microsatellite DEMETER and on the ground at Langmuir. Analyses of ELF/VLF measurements associated with sprite events observed on 28 July 2005 and 3 August 2005 are presented

In situ measurements of both energetic particles and VLF waves have been carried out in a joint program involving the Japanese satellite EXOS-B and the Siple Station VLF transmitter. A general description of the experiment is given as well as some results concerning wave-particle correlations. Telemetry data aquisition was made by NASA tracking stations during summer in 1979 and during the succeeding winter. This report describles mainly the results of the summer experiment. The Siple signal was detected on about 32% of the observed passes. Siple triggered emissions (ASE) were observed on five passes concentrated in a period from August 14 to 18, while it was geomagnetically quiet after a large magnetic storm on August 13. There were two types of Siple ASE; one is due to a direct triggering, which is normally found in the ground observation, and the other is one triggered by echoing signals, which is not popularly found on the ground. Siple signals triggered ASE when the signal frequency was around half the equatorial cyclotron frequency and electron flux was large for all observed energy channels. The observed delay time of the Siple signal from the transmitter to the satellite can be interpreted by either ducted or non-ducted propagation., 資料番号: SA0125927000