Dominique Lagoutte

Further evidence of triggering chorus emissions from wavelets in the hiss band

Abstract The results of detailed spectral analyses and direction finding for ELF data containing simultaneous hiss and chorus have been reported on the basis of the data observed on-board GEOS 1 satellite in the outer magnetosphere at a high geomagnetic latitude (Λ ∼- 25°). A combination of the present results with those from our previous equatorial studies has yielded the following findings : (1) each chorus element has a tendency to originate from the hiss band and is asymptotic to the hiss band ; (2) the intensity and occurrence of the chorus are closely correlated with the intensity of the underlying hiss ; (3) the hiss band exhibits structures or wavelets (i.e. monochromatic wave component with significant duration) and we notice the existence of the causative wavelet at the foot of each chorus element ; (4) when we expect triggering chorus from a wavelet near the upper edge of the hiss band, the duration of the wavelet increases with decreasing intensity ; (5) very similar φ (azimuthai angle) values are noticed for both the hiss and chorus, suggesting that both phenomena come from the same source region. These observational facts might lead us to conclude that a wavelet existing near the upper edge of the hiss band is able to generate a chorus emission through coherent wave-particle interaction in the outer magnetosphere, as in the case of active VLF wave injection experiments.

On the use of tricoherent analysis to detect non-linear wave-wave interactions

Abstract For the description of non-linear effects in dispersive media, the approach of three- and four-wave interactions may be used. Higher-order spectral analysis is required for identifying interacting waves in random signals. The bicoherence spectrum, which is the normalized cumulant spectrum of the third order, has been applied to numerous studies for analysing three-wave interactions in numerical, laboratory and natural experiments. Here, we develop the Incoherence spectrum to detect four-wave interactions, calculate statistical properties of the trispectrum and tricoherence estimators as well as the confidence interval for the latter, and present results of numerical estimations of the tricoherence using synthetic signals.

Unexpected Very Low Frequency (VLF) Radio Events Recorded by the Ionospheric Satellite DEMETER

DEMETER was a low Earth orbiting microsatellite in operation between July 2004 and December 2010. Its scientific objective was the study of ionospheric perturbations in relation to seismic activity and man-made activities. Its payload was designed to measure electromagnetic waves over a large frequency range as well as ionospheric plasma parameters (electron and ion densities, fluxes of energetic charged particles). This paper will show both expected and unusual events recorded by the satellite when it was in operation. These latter events have been selected from the DEMETER database because they are rare or even have never been observed before, because they have a very high intensity, or because they are related to abnormalities of the experiments under particular plasma conditions. Some events are related to man-made radio waves emitted by VLF ground-based transmitters or power line harmonic radiation. Natural waves, such as atypical quasi-periodic emissions or uncommon whistlers, are also shown.

Rosetta spacecraft influence on the mutual impedance probe frequency response in the long Debye length mode

Abstract During the Rosetta flyby of comet 46P/Wirtanen from 2011, plasma observations will be obtained from a number of instruments, the mutual impedance probe (MIP) is one of them. The mutual impedance technique is based on the measurements, as a function of the frequency, of the quasi-electrostatic coupling between an emitter and two receivers separated from the emitted source by at least twice the local plasma Debye length. The electron number density is deduced from the electron plasma frequency at which the transfer function reaches its maximum, and the Debye length, λD, is deduced from the positions of the minima above the plasma frequency. The long Debye length (LDL) mode, which operates in the 7–168 kHz range, is a secondary mode of MIP that has been designed to probe cometary plasmas when λD is longer than 70 cm. In that case, the Rosetta spacecraft presence cannot be neglected due to its conductive structures, the dimensions of which are of order of the emitter–receiver distance. A numerical simulation of the LDL mode is then necessary. The discrete surface charge distribution (DSCD) method, which is well adapted to the electric antenna problems in a kinetic plasma, would be suitable for the Rosetta flyby measurements. Here, all the conductive surfaces (spacecraft, solar panels and antennae) are compared with an alternating charge distribution that contributes to the LDL mode transfer function. The preliminary results show that in the early parts of the Rosetta mission, the cometary plasma can reasonably be considered to be Maxwellian, homogeneous, isotropic, collisionless and unmagnetized in the range of λD from 0.7 to 2.5 m. The numerical results are compared with those obtained by ignoring the spacecraft influence. It appears that the resonance peak at the plasma frequency is sharpest and strongest when the spacecraft influence is considered. Moreover, the antiresonance frequencies which occur on both sides of the plasma frequency depend on the Debye length of the surrounding plasma. Hence, the LDL mode should be able to measure the electron temperature in the range from about 104– 2×10 5 K . Plasmas with electron number densities lying between 22 and 180 cm −3 should also be probed. This will allow the LDL mode to distinguish the boundary between the Wirtanens cometary plasma and the Solar Wind in the prime investigations of the Rosetta mission, and this will complete the measurements made by the principal MIP mode in the non-magnetic cavity of the comet.

Observation of nonlinear interactions in large-scale density enhancements of the high-latitude ionosphere

Simultaneous measurements of electron density and ELF electric field by the AUREOL-3 satellite have allowed Cerisier et al. [1985] to point out the existence of large-scale density irregularities in the high-latitude topside F region and to establish the role of the gradient drift instability in the plasma structuring. High-resolution spectra of the density and field fluctuations show the presence of low-frequency peaks linked by relationships such that ƒm + ƒn = ƒm+n. Bispectral analyses point out nonlinear interactions between waves at 7.8 and 15.6 Hz and density irregularities at 5.85 Hz. The consistency of the results with regard to instability models is discussed. The existence of an interaction process involving Schumann resonances is suggested.

The Experimental Results of triggering Chorus Emissions from Monochromatic Wave Components in the Hiss Band in the Outer Magnetosphere

The results of detailed spectral analyses and direction finding for both VLF and ELF data containing simultaneous hiss and chorus are reported on the basis of the data observed onboard the GEOS1 satellite. The following important results are obtained; (1)Each chorus element has a tendency to be originated from the hiss band and is asymptotic to the hiss band. (2)The intensity and occurrence of chorus are closely correlated with the intensity of the underlying hiss. (3)The hiss has been so far considered to be random and incoherent, but it exhibits some structures or wavelets, which are monochromatic wave components with significant duration, and we can notice the exsistence of the causative wavelet at the foot of each chorus element. (4)When we expect triggering a chorus from a wavelet at the upper edge of the hiss band, the duration of the wavelet increases with decreasing its intensity. (5)Very similar φ values of the azimuth are noticed for both hiss and chorus, suggesting that both phenomena come from the same source region. These experimental facts might lead us to conclude that a wavelet existing at the upper edge of the hiss band is able to generate a chorus emission through the coherent wave-particle interaction in the outer magnetosphere as in the case of active VLF wave injection experiments.

Electron density measurement in space plasma, comparison between two techniques: Sounder by relaxation and mutual impedance probe

Summary form only given. In space plasma, the determination of the electron density is an important parameter to understand the wave propagation and energy transfers between wave and particles. Usually, the plasma density and debye length are much different in solar wind and planetary environment than laboratory plasma and it is needed to sound large volume. Several ways exist to estimate the electron density that can separate into two main categories: the passive and active techniques. In this presentation, we propose to discuss the active techniques and we describe on sounder by relaxation instrument and mutual impedance measurement. The both types consist to emit in the plasma a monochromatic frequency. Then, plasma response is recorded for several frequencies, scanning a frequency range including the characteristic plasma frequencies like plasma frequency Fpe, electron gyro-frequency Fce, gyro-harmonics Fcen and bersteins modes Fqn which are perpendicular electrostatic resonances. However, the way to record and the energy used are completely different following the technique. Sounder by relaxation use a powerful pulse which excites the surrounding plasma and the plasma response is listened after a short delay (few milliseconds). When the emitted frequency fit the plasma resonances, the recorded frequency will be seen with a large amplitude and allow us to identify the plasma resonances whose derive the electron density and magnetic field magnitude. This technique is very efficient but the transmitted energy is above the thermal energy and perturbs strongly the electron distribution function. At the difference, the mutual impedance probes technique the transmitting and receiving frequency is done at the same time and it is the plasma permittivity which is measured. This technique needs to use simultaneously two antennas. The interpretation must take into an account the influence of the conductive structures of the satellite and numerical simulation is needed. But, the emitted energy is less than the thermal plasma energy and perturbations are negligible. We will show examples WHISPER sounder measurement on board CLUSTER/ESA mission and we will illustrate the mutual impedance technique by examples done by Rosetta mission in earth magnetosphere.

On the Experimental Results of Hiss-Triggered Chorus Observed Onboard GEOS-1 Satellite in the Outer Magnetosphere

The co-existence of hiss and chorus is frequently observed onboard satellites in the outer magnetosphere. The purpose of this paper is to investigate experimentally the association between hiss and chorus and to clarify the role of hiss in chorus generation. The paper is based on the detailed spectral analyses and direction finding measurements for the simultaneous occurrence of hiss and chorus observed onboard GEOS-1 satellite in the outer magnetosphere. The event analyzed here is observed near the equator (geomagnetic latitude, 6.8–8.5 degrees). Then, we propose the mechanism of hiss-triggered chorus based on our experimental findings.