F. Lefeuvre

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.

Characteristics and conditions of production of transient luminous events observed over a maritime storm

On the night of 15/16 November 2007, cameras in southern France detected 30 transient luminous events (TLEs) over a storm located in the Corsican region (France). Among these TLEs, 19 were sprites, 6 were halos, and 5 were elves. For 26 of them, a positive “parent” cloud‐to‐ground lightning (P+CG) flash was identified. The peak current of the P+CG flashes for the sprites had an average value of 63 kA and had a maximum value of 125 kA. The flashes for the halos and the elves had average values of 272 and 351 kA, respectively, and they had maximum values of 312 and 384 kA, respectively. No TLEs were detected after negative CG flashes with very large peak currents. Among the 26 P+CG flashes, 23 were located in a stratiform region with reflectivity values lower than 45 dBZ. The CG flashes in this region were classified into two groups according to the time interval separating them from the following flash: one group with values less than 2 s and one with values greater than 2 s. About 79% of all CGs were produced in a sequence of at least two flashes less than 2 s apart. For 65.5% of the sequences, the first flash was positive with an average peak current of 73 kA, while the later +CG flashes in a sequence had much lower peak currents. Several triangulated sprites were found to be shifted from their P+CG flashes by about 10 to 50 km and preferentially downstream. The observations suggest that the P+CG flashes can initiate both sprites and other CG flashes in a storm.

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.

On remote sensing of transient luminous events' parent lightning discharges by ELF/VLF wave measurements on board a satellite

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 lEnvironnement et de lEspace (LPCE). TLEs are optically observed from the U.S. Langmuir Laboratory, while ELF/VLF waveform data are simultaneously recorded on board the Centre National dEtudes 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

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°).

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.

Asymmetric V‐shaped streaks recorded on board DEMETER satellite above powerful thunderstorms

[1]xa0We report here observations of both symmetric and asymmetric forms of V-shaped streaks on VLF spectrograms recorded on board the DEMETER satellite. Recent investigations have shown that V-shaped streaks are associated with intense and numerous 0+ whistlers generated in the VLF range by active thunderstorms. To understand the origin of the different spectral forms of the V-shaped events, a systematic survey of these phenomena is performed by means of a visual inspection of the VLF spectrograms covering more than 5 years of DEMETER observations. Asymmetric events are more frequently observed over high-latitude regions. The influence of the magnetic field inclination on the observed asymmetry is investigated. First, a full wave method is used to compute, at the lower ionospheric boundary, the electromagnetic pattern that is due to a streak of lightning. This allows us to show a small asymmetry especially for high frequencies (f > 10 kHz). Then, cold plasma dispersive properties are used to determine the characteristics of the waves propagating from the mesosphere to the satellite altitude. We find that, at the plasma cutoff altitude, the propagation is more efficient for waves having wave-normal directions parallel, or quasi parallel, to the Earths magnetic field direction. Finally, a detailed analysis of an asymmetric V-shaped event is performed. The precise localization of the lightning associated with the V-shaped event is provided by the METEORAGE network. A relationship between the local inclination of the magnetic field above active thunderstorms and the asymmetry of the V-shaped event is pointed out for this particular event.

An unusual VLF signature structure recorded by the DEMETER satellite

A type of electromagnetic phenomenon has been found in the electric VLF data measured by the low Earth orbit DEMETER satellite, which was nonidentified earlier as a different class of electromagnetic VLF events. The phenomenon, termed as swallow‐ tailed whistler (STW) after its shape, seems to be similar to a whistler, but following the main trace, an additional trace appears with monotonously increasing frequency. The secondary trace, lasting less than 80 ms within the recorded 20 kHz bandwidth joins at a given Starting Furcation Frequency. In a 7 month long time interval three series of strong STWs were found in a geographically confined search zone. Further, 10 weak STW periods have been identified by a thorough review of a 2 month long recording. Several STWs were found by the investigation of randomly selected DEMETER burst VLF recording acquired globally. On the basis of comparisons with previous studies, we can exclude that this phenomenon is generated by plasma processes in the vicinity of the satellite though the formation mechanism of this (ionospheric) signal is so far unclear. It is possible that this event type appeared in earlier records too, however, without identification.