J. L. Rauch

Complete wave‐vector directions of electromagnetic emissions: Application to INTERBALL‐2 measurements in the nightside auroral zone

We present several newly developed methods for wave propagation analysis. They are based on simultaneous measurement of three magnetic field components and one or two electric field components. The purpose of these techniques is to estimate complete wave vector direction and the refractive index. All the analysis results are validated by well defined simulated data. Propagation analysis of natural emissions in the night-side auroral zone at high altitudes is done using the data of the MEMO (Mesures Multicomposantes des Ondes) experiment onboard INTERBALL-2. The results show that a bursty whistler mode emission propagates toward the Earth near the resonance cone. Upward propagating auroral kilometric radiation in the R-X mode represents another example demonstrating the potential of such analysis for future applications.

Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density

[1] Chorus emissions are generated by a nonlinear mechanism involving wave‐particle interactions with energetic electrons. Discrete chorus wave packets are narrowband tones usually rising (sometimes falling) in frequency. We investigate frequency sweep rates of chorus wave packets measured by the Wideband data (WBD) instrument onboard the Cluster spacecraft. In particular, we study the relationship between the sweep rates and the plasma density measured by the WHISPER active sounder. We have observed increasing values of the sweep rate for decreasing plasma densities. We have compared our results with results of simulations of triggered emissions as well as with estimates based on the backward wave oscillator model for chorus emissions. We demonstrate a reasonable agreement of our experimental results with theoretical ones. Citation: Macusova, E., et al. (2010), Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density,

Active and passive plasma wave investigations in the earth's environment: The cluster/whisper experiment

Abstract The Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation, WHISPER, performs high frequency electric field measurements on the four satellites of the CLUSTER mission. In active mode, the WHISPER behaves like a classical topside sounder. It provides, via the identification of resonances that are excited at characteristic frequencies of the encountered plasmas reliable and accurate determination of the total electron density and magnetic field strength. Whenever the transmitter is switched off, the WHISPER becomes a simple wave receiver (passive mode). The 2 to 80 kHz frequency range of the electric component of natural waves is then monitored. The main objective of the presentation is to highlight the plasma and natural waves diagnosis capabilities of the WHISPER instrument. The way the plasma resonances are extracted and identified is pointed out and results obtained from the solar wind down to the Earths plasmasphere are shown.

DEMETER Observations of EM Emissions Related to Thunderstorms

The paper is related to specific emissions at frequency <3 MHz observed by the low altitude satellite DEMETER in relation with the thunderstorm activity. At its altitude (∼700 km), the phenomena observed on the E-field and B-field spectrograms recorded by the satellite are mainly dominated by whistlers. Particular observations performed by DEMETER are reported. It concerns multiple hop whistlers and interaction between whistlers and lower hybrid noise. Two new phenomena discovered by the satellite are discussed. First, V-shaped emissions up to 20 kHz are observed at mid-latitude during night time. They are centered at the locations of intense thunderstorm activity. By comparison with VLF saucers previously observed by other satellites in the auroral zones it is hypothesized that the source region is located below the satellite and that the triggering mechanism is due to energetic electrons accelerated during sprite events. Second, emissions at frequency ∼2 MHz are observed at the time of intense whistlers. These emissions are produced in the lower ionosphere in probable relation with Transient Luminous Events (TLEs).

Propagation characteristics of dayside low‐altitude hiss: Case studies

A comprehensive set of wave analysis techniques is applied to dayside ELF hiss observed over three typical orbits of the low-altitude AUREOL 3 satellite. Validity domains are established for each technique, whose results indicate the following propagation characteristics. Within the plasmasphere the waves are narrow-band, have a lower cutoff frequency close to the local proton gyrofrequency ƒH+, and propagate downward with very oblique k vectors directed toward lower L values. Left-hand mode waves are detected just below ƒH+. Within the plasmapause gradient the waves are broad-band, have a lower cutoff frequency at the vicinity of ƒH+, and propagate mainly upward with oblique k vectors directed toward lower L values. Within the light-ion trough region and the auroral zones the waves are broad-band, have lower cutoff frequencies that can be below the local ƒH+, and propagate downward with k vectors along B0. Narrowband emissions detected in the vicinity of the cusp propagate upward. Exceptions are found at frequencies just above ƒH+ where, at nearly all invariant latitudes, waves are commonly upgoing. We conclude that (1) the reflection at the two-ion cutoff frequency is a very efficient process for returning waves to higher altitudes and (2) the large density gradients that characterize the plasmapause and the cusp seem to act as traps for waves reflected below the satellite.

Positive ion distributions in the morning auroral zone: Local acceleration and drift effects

Abstract We report on the typical structure of the large scale ion precipitation in the morning sector of the auroral zone and associated low frequency electromagnetic waves. Data obtained during near radial passes of the AUREOL-3 satellite point to a distinction between two main precipitation regions: 1) In the poleward part of the auroral zone the latitudinal variation of the average energy (or temperature) of the precipitated ions (mainly H+) indicate that they are adiabatically accelerated in the outer magnetosphere. This “high energy” (⋍ 3 to > 20 keV) precipitation is usually associated with a low energy (E 3 hours) we suggest that the precipitation of ionospheric ions inside the diffuse aurora results from convection and corotation of the ions accelerated to suprathermal energies at higher latitudes.

The WHISPER Relaxation Sounder and the CLUSTER Active Archive

The Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) instrument is part of the Wave Experiment Consortium (WEC) of the CLUSTER mission. With the help of the long double sphere antennae of the Electric Field and Wave (EFW) instrument and the Digital Wave Processor (DWP), it delivers active (sounding) and natural (transmitter off) electric field spectra, respectively from 4 to 82 kHz, and from 2 to 80 kHz. These frequency ranges have been chosen to include the electron plasma frequency, which is closely related to the total electron density, in most of the regions encountered by the CLUSTER spacecraft. Presented here is an overview of the WHISPER data products available in the CLUSTER Active Archive (CAA). The instrument and its performance are first recalled. The way the WHISPER products are obtained is then described, with particular attention being paid to the density determination. Both sounding and natural measurements are commonly used in this process, which depends on the ambient plasma regime. This is illustrated using drawings similar to the Bryant plots commonly used in the CLUSTER master science plan. These give a clear overview of typical density values and the parts of the orbits where they are obtained. More information on the applied software or on the quality/reliability of the density determination can also be highlighted.

Propagation characteristics of auroral kilometric radiation observed by the MEMO experiment on Interball 2

The MEMO experiment is a part of the Interball 2 wave consortium. It is connected to a total of six electric and nine magnetic independent sensors. It provides waveforms associated with the measurement of two to five components in the three frequency bands: ELF (5–1000 Hz), VLF (1–20 kHz), and LF (20–250 kHz). Waveforms of three magnetic components and one electric component recorded during observations of auroral kilometric radiation (AKR) allow a detailed study of the characteristics of these emissions. In particular, the wave normal directions of AKR relative to the Earths magnetic field are determined using several methods: the classical methods based on the plane wave approximation [Means, 1972] and the wave distribution function method which represents the evaluation of the wave energy density distribution with respect to the angular frequency and the wave normal direction(s). One event is fully analyzed in this paper. It is shown that AKR propagates with a polarization quasi-circular (ellipticity value ∼ 0.9), a right polarization (i.e., R-X mode), and wave normals weakly oblique (∼30°).

The Whisper Relaxation Sounder Onboard Cluster: A Powerful Tool for Space Plasma Diagnosis1, 2 around the Earth

The WHISPER relaxation sounder that is onboard the four CLUSTER spacecraft has as its main scientific objectives to monitor the natural waves in the 2 kHz–80 kHz frequency range and, mostly, to determine the total plasma density from the solar wind down to the Earths plasmasphere. To fulfill these objectives, the WHISPER uses the two long double sphere antennae of the Electric Field and Wave experiment as transmitting and receiving sensors. In its active working mode, the WHISPER works according to principles that have been worked out for topside sounding. A radio wave transmitter sends an almost monochromatic and short wave train. A few milliseconds after, a receiver listens to the surrounding plasma response. Strong and long lasting echoes are actually received whenever the transmitting frequencies coincide with characteristic plasma frequencies. Provided that these echoes, also called resonances, may be identified, the WHISPER relaxation sounder becomes a reliable and powerful tool for plasma diagnosis. When the transmitter is off, the WHISPER behaves like a passive receiver, allowing natural waves to be monitored. This paper aims mainly at the resonance identification process description and the WHISPER capabilities and performance highlighting.

Propagation of Z‐mode and whistler‐mode emissions observed by Interball 2 in the nightside auroral region

A case study of VLF (very low frequency) natural emissions propagating at high altitude above the nightside auroral zone is presented. The analysis is based on data of the MEMO experiment on board the Interball 2 spacecraft. In its VLF band the device records waveforms of three magnetic and two electric field components covering a frequency range 1–20 kHz. Several analysis methods are applied in order to obtain the details on the wave propagation and mode structure and to identify possible source regions. We demonstrate that observed band-limited emission consists of Z-mode and whistler-mode waves. Downgoing Z-mode waves are found just above the lower cutoff at the L=0 frequency. A possible source may be connected with the electron cyclotron resonance mechanism taking place in the nightside sector at altitudes above 21,000 km. Upgoing whistler-mode waves with upper cutoff at the local plasma frequency are probably generated by upgoing electrons in the auroral region at altitudes lower than the actual satellite position, i.e., below 18,000 km.