M. Parrot

The Cluster Spatio-Temporal Analysis of Field Fluctuations (Staff) Experiment

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10-3 nT Hz-1/2 and 3 × 10-5 nT Hz-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.

Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss

We interpret observations of low-altitude electromagnetic ELF hiss observed on the dayside at subauroral latitudes. A divergent propagation pattern has been reported between 50° and 75° of geomagnetic latitude. The waves propagate with downward directed wave vectors which are slightly equatorward inclined at lower magnetic latitudes and slightly poleward inclined at higher latitudes. Reverse ray tracing using different plasma density models indicates a possible source region near the geomagnetic equator at a radial distance between 5 and 7 Earth radii by a mechanism acting on highly oblique wave vectors near the local Gendrin angle. We analyze waveforms received at altitudes of 700–1200 km by the Freja and DEMETER spacecraft and we find that low-altitude ELF hiss contains discrete time-frequency structures resembling wave packets of whistler mode chorus. Emissions of chorus also predominantly occur on the dawnside and dayside and have recently been considered as a possible source of highly accelerated electrons in the outer Van Allen radiation belt. Detailed measurements of the Cluster spacecraft at radial distances of 4–5 Earth radii show chorus propagating downward from the source region localized close to the equator. The time-frequency structure and frequencies of chorus observed by Cluster along the reverse raypaths of ELF hiss are consistent with the hypothesis that the frequently observed dayside ELF hiss is just the low-altitude manifestation of natural magnetospheric emissions of whistler mode chorus.

VLF Electromagnetic Waves Observed Onboard GEOS-1

This paper is concerned mainly with the information which can be extracted from frequency-time spectra in the VLF range. The instrument used is the correlator which has a good frequency resolution (50 Hz) and time resolution (30 ms) in one magnetic and one electric component simultaneously. By suitable computer analysis, it is possible for instance to distinguish between the two dominant electromagnetic emissions, hiss and chorus, as well as to display the complete spectra. This treatment is applied to the Survey periods, which are a fixed sequence of modes, repeated every hour on the hour in order to have reference data from GEOS analogous to many ground-based observatories. One result of this treatment obtained already is that hiss and chorus normally appear together, although one or the other may be dominating in intensity. The occurrence rate of these emissions in local time is also given.For continuous surveillance the filterbank data are used. There are 16 frequency filters supplying magnetic and electric amplitude at few different frequencies. Using these data, a storm sudden commencement can be followed with good time resolution (1 s), and an interesting correlation has been found in a few cases between the VLF signal amplitude and the cold plasma density (as measured by the active part of the S-300 experiment).

Interpretation of Cluster data on chorus emissions using the backward wave oscillator model

The measurements of chorus emissions by four closely separated Cluster spacecraft provide important information concerning the chorus generation mechanism. They confirm such properties of the wave source as their strong localization near the equatorial cross section of a magnetic flux tube, an almost parallel average wave-vector direction with respect to the geomagnetic field, and an energy flux direction pointing outward from the generation region. Inside this region, Cluster discovered strong temporal and spatial variations in the amplitude with correlation scale lengths of the order of 100 km across the magnetic flux. The wave electric field reached 30 mV/m, and the maximum growth and damping rates are of the order of a few hundreds of s−1. These and other properties of the detected chorus emissions are discussed here in relation with the backward wave oscillator mechanism. According to this mechanism, a succession of whistler wave packets is generated in a small near-equatorial region with temporal an...

Statistical study of ELF/VLF emissions recorded by a low‐altitude satellite during seismic events

This paper presents a statistical study of low-altitude satellite data linked to seismic events. Data of the satellite AUREOL 3 are considered during 24-hour windows around the time of 325 earthquakes with Ms > 5. Amplitudes at the output of different frequency filters in the ELF/VLF range, connected to electric or magnetic components, were stored in a file. This file is organized as functions of three parameters: Δt, the difference between the time of the earthquake and the time of the satellite data; ΔLong, the difference between the longitude of the earthquake and the longitude of the satellite; and ΔInv.Lat, the difference between the invariant latitude of the earthquake and the invariant latitude of the satellite. When all data are averaged over time, the amplitude is maximum in the interval ΔLong < 10° regardless of ΔInv.Lat. This indicates that, due to propagation, waves related to seismic processes can be observed all along the magnetic meridian passing over the epicenter of an earthquake. However, as the AUREOL 3 data were not continuously recorded, thus providing a limited number of events, it was impossible to carry out a complete statistical study.

Electrostatic discharge in Martian dust storms

Although the Martian atmosphere does not satisfy general requirements for lightning generation, there is a possibility of electrical discharge in the case of strong surface winds and a resulting extremely large dust mass loading in the course of large dust storms occurring on this planet. On Earth, negative potential gradients of many thousands of volts per meter have been measured during dust storms when winds are sufficiently strong. However, owing to a lower pressure in the Martian atmosphere, the required voltage for electrical breakdown is lower than on Earth. After a brief review of the observations concerning Earths atmosphere and the laboratory experiments performed to understand these phenomena, the results of numerical simulation of the electrification in Martian dust storms are presented. Known characteristics of Martian dust grains and the Martian atmosphere are considered, and the different forces applied to the dust particles are taken into account. The electrostatic potential and the evolution of the charge density are calculated using Poissons equation and the continuity equation. Two different wind configurations in the Martian atmosphere are studied: a wind vortex and a wind parallel to the ground encountering an obstacle (a small hill). It is shown that electric discharge occurs under specific conditions.

Electromagnetic ELF radiation from earthquake regions as observed by low‐altitude satellites

Seismo-Electromagnetic (SEM) waves observed by low-altitude satellites passing over seismic regions were studied. The data of the COSMOS-1809 satellite were analysed over the earthquake region in Armenia during the period from January 20 to February 17, 1989. Intense EM radiation at frequencies below 450 Hz was observed at the L-shells of the earthquake, during 12 orbits out of the 13 that passed within 6° in longitude from the epicenter, and during 1 out of 6 in the range of 6°–8° longitude away from this region. The other orbits, which passed 10°–12° from the epicentre, showed no effect. To complete this study, we used the emissions observed by another low-altitude satellite (AUREOL-3).It is shown that during the event the seismic region is permanently radiating; the intensity and the envelope shape of the wave depend on its time relatively to the time of the earthquake. Their frequency spectra are compared to the average spectrum recorded in the same geomagnetic regions. Similar wave intensities and spectral distributions were observed on the two satellites during the seismic periods.

Radiation belt electron precipitation due to VLF transmitters: Satellite observations

In the Earth’s inner magnetosphere, the distribution of energetic electrons is controlled by pitch-angle scattering by waves. A category of Whistler waves originates from powerful ground-based VLF transmitter signals in the frequency range 10–25 kHz. These transmissions are observed in space as waves of very narrow bandwidth. Here we examine the significance of the VLF transmitter NWC on the inner radiation belt using DEMETER satellite global observations at low altitudes. We find that enhancements in the 100–600 keV drift-loss cone electron fluxes at L values between 1.4 and 1.7 are linked to NWC operation and to ionospheric absorption. Waves and particles interact in the vicinity of the magnetic equatorial plane. Using Demeter passes across the drifting cloud of electrons caused by the transmitter; we find that 300 times more 200 keV electrons are driven into the drift-loss cone during NWC transmission periods than during non-transmission periods. The correlation between the flux of resonant electrons and the Dst index shows that the electron source intensity is controlled by magnetic storm activity.

Propagation analysis of plasmaspheric hiss using Polar PWI measurements

We have analyzed high-rate waveform data, taken by the POLAR Plasma Wave Instrument at high altitudes in the equatorial plasmasphere, to study plasmaspheric hiss in the range of frequencies between 100 Hz and several kHz. These emissions are found almost everywhere in the plasmasphere, and their origin is still controversial. Our analysis of several cases shows that most of the waves were propagating more or less parallel to the Earths magnetic field, but sometimes a few of them were propagating obliquely with their normals near the Gendrin angle. Evidence of amplification was found near the geomagnetic equator. The results suggest that waves with normals both parallel and anti-parallel to the magnetic field were being amplified by the classical mechanism that involves gyroresonant interaction with energetic electrons.

Decrease of intensity of ELF/VLF waves observed in the upper ionosphere close to earthquakes: A statistical study

[1] We present results of a systematic study of intensity of VLF electromagnetic waves observed by the DEMETER spacecraft in the upper ionosphere (altitude 700 km). We focus on the detailed analysis of the previously reported decrease of wave intensity shortly before the main shock during the nighttime. Using a larger set of data (more than 3.5 years of measurements) and a newly developed data processing method, we confirm the existence of a very small but statistically significant decrease of wave intensity 0‐4 hours before the time of the main shock at frequencies of about 1.7 kHz. It is shown that the decrease does not occur directly above the earthquake epicenter but is shifted about 2 in the westward direction. Moreover, it is demonstrated that the decrease occurs more often close to shallower earthquakes and close to earthquakes with larger magnitudes, as it is ‘‘intuitively’’ expected, representing an additional proof of validity of the obtained results. Finally, no dependence has been found on the occurrence of the earthquake below the ocean or below the continents.