Analysis of fine ELF wave structures observed poleward from the ionospheric trough by the low-altitude satellite DEMETER

International audienceDEMETER was a three-axis stabilized Earth-pointing spacecraft launched on 29 June 2004 into a low-altitude (710 km) polar and circular orbit that was subsequently lowered to 650 km until the end of the mission in December 2010. DEMETER measured electromagnetic waves all around the Earth except at magnetic invariant latitudes >65°. The frequency range for the electric field was from DC up to 3.5 MHz and for the magnetic field from a few hertz up to 20 kHz. Electromagnetic ion cyclotron (EMIC) waves have been previously observed by DEMETER close to the ionospheric trough during high magnetic activity, and this paper describes another type of EMIC waves. These waves are also observed close to the trough, but they extend poleward, with the trough acting as a boundary. They are observed exclusively during the night and preferentially during geomagnetic substorms. The analysis of wave propagation shows that they propagate nearly along the ambient magnetic field and that they come from larger radial distances

LOFAR Observations of Lightning Initial Breakdown Pulses

This paper reports an observational study of lightning initial breakdown pulses (IBPs) using the low-frequency array radio telescope and a broadband magnetic field sensor. The data show that the overall spatiotemporal evolution of the electrical breakdown causing an IBP is rather complex. During an IBP, spatially and temporally separated bursts of very high frequency (VHF) electromagnetic radiation occur in a volume on the order of 1003 m3, and they are coincident with brief magnetic field pulses, indicating that the location of the active breakdown region can change suddenly. Furthermore, recurrent breakdown activity is observed, especially at the location of the VHF burst. Interpreting each VHF burst as being generated by a corona burst, an IBP pulse appears to start off from an initial corona burst and subsequent corona bursts enhance it. We further suggest that the generation of IBPs likely involves multiple space stems/leaders and connections between them

Spectral features of lightning-induced ion cyclotron waves at low latitudes: DEMETER observations and simulation

International audience[1] We use a comprehensive analysis of 6-component ELF wave data from the DEMETER satellite to study proton whistlers, placing emphasis on low-latitude events originating from lightning strokes in the hemisphere opposite to the hemisphere of observation. In this case, the formation of proton whistlers does not involve mode conversion caused by a strong mode coupling at a crossover frequency, although a polarization reversal remains an important element in formation of the phenomenon. DEMETER measurements of the six electromagnetic field components in the frequency band below 1000 Hz make it possible to determine not only the dynamic spectrum, but also the wave polarization, the wave normal angle, and the normalized parallel component of the Poynting vector. This permits us to address fine features of proton whistlers, in particular, we show that the deviation of the upper cutoff frequency from the equatorial cyclotron frequency is related to the Doppler shift. Experimental study of proton whistlers is supplemented by an investigation of ion cyclotron wave propagation in a multicomponent magnetoplasma and by numerical modeling of spectrograms, both in the frame of geometrical optics

Case studies on the wave propagation and polarization of ELF emissions observed by Freja around the local proton gyrofrequency

International audienceUsing a multicomponent measurement of electric and magnetic fields in the ELF range, we investigate the polarization and propagation characteristics of several types of electromagnetic emissions around the local proton gyrofrequency. The data of the loworbiting Freja satellite are examined in the auroral and subauroral regions as well as inside the plasmasphere. We confirm previous results on the properties of the high-latitude hiss with a sharp lower cutoff just below the local proton gyrofrequency. The waves propagate downward from higher altitudes outside the plasmasphere and reflect at the local two-ion cutoff frequency. With an enhanced data representation we are able to characterize the reflected upgoing waves on both poleward and equatorward edges of the hiss emission. The high-latitude emissions in a large band below the local proton gyrofrequency contain right-hand circularly polarized waves propagating nearly parallel to the terrestrial magnetic field. We believe these emissions can originate by tunnelling of the right-hand-polarized hiss below the two-ion crossover frequency. We report high-latitude "frequency drift phenomena" below the local proton gyrofrequency. The frequency of these faint emissions increases by several tens of hertz per latitude degree. The electromagnetic noise at low frequencies is always found with a high degree of a nearly linear polarization. Upgoing ion whistlers are observed to follow each other with time delays as low as about 100 ms

Properties of unipolar magnetic field pulse trains generated by lightning discharges

International audienceWe analyze and describe trains of regular unipolar microsecond-scale magnetic field pulses produced by intracloud lightning discharges. Waveforms of the magnetic field are measured using a newly developed broadband analyzer with a sampling interval of 12.5 ns. The observed trains contained several tens of pulses, and the time interval between neighboring pulses typically varied between 1 and 10 ms. The amplitude of the pulses also varied by 1 order of magnitude. A systematic analysis of these variations is done for the first time. The interpulse interval is usually increasing (on average by 4.1 ms during a train), and the pulse amplitude is decreasing (on average by 15% of the maximum value within a given train). We propose a possible generation mechanism based on a hypothesis that periodical charge structures are present in the thundercloud. This mechanism can explain the observed evolution of peak amplitudes and interpulse intervals

On the upper frequency limit of whistler mode waves observed by low-altitude spacecraft

International audienceFrequency-latitude plots of electromagnetic wave intensity in the very low frequency range (VLF, up to about 20 kHz) observed by the low altitude DEMETER spacecraft are analyzed. Apart from electromagnetic waves generated by plasma instabilities in the magnetosphere, a significant portion of the detected wave intensity comes from ground-based lightning activity and VLF military transmitters. These whistler mode waves are observed not only close to source locations, but also close to their geomagnetically conjugated points. There appears to be an upper frequency limit of such emissions, where the wave intensity substantially decreases. Its frequency roughly corresponds to half of the equatorial electron cyclotron frequency at a respective magnetic field line, suggesting a relation to wave ducting in ducts with enhanced density. However, it seems to exhibit a non-negligible longitudinal dependence and it is different during the day than during the night. We use a realistic model of the Earth's magnetic field to explain the observed variations. We interpret the observations in terms of ducted/unducted wave propagation, and we compare the wave intensities in the source hemisphere with those measured in the hemisphere geomagnetically conjugated

Analysis of subprotonospheric whistlers observed by DEMETER: A case study

International audience[1] Subprotonospheric (SP) whistlers consist of a series of low‐dispersion components that result from repeated reflections between the base of the ionosphere and altitudes up to ∼1000 km. We have used wave‐normal angles and plasma characteristics measured by the DEMETER microsatellite as an input for a three‐dimensional ray tracing technique. For several SP whistlers we have also succeeded in finding the causative lightning located at relatively large distances from the satellite footprint along the geomagnetic field line. We show that the reflections and formation of the SP whistlers take place owing to an oblique propagation, with respect to the magnetic field, in the waveguide formed by a profile of the increasing lower hybrid resonance frequency in the upper ionosphere and the base of the ionosphere. We have observed propagation across the magnetic meridian planes. We conclude that the individual components of the SP whistler propagate along different raypaths. The reflected components enter the ionosphere at relatively large distances from the satellite footprint and experience a spread of wave‐normal angles during this entry. Depending on the initial wave‐normal angle, these waves undergo a different number of reflections before reaching the satellite, thus arriving with different time delays. However, the first component observed of a SP whistler is formed by waves entering the ionosphere at relatively small distances from the satellite footprint and at relatively small wave‐normal angles. These waves do not reflect above the satellite but propagate to the opposite hemisphere

Chorus and chorus-like emissions seen by the ionospheric satellite DEMETER

International audienceA lot of different emissions have been detected by the low-altitude satellite DEMETER (Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions), and the aim of this paper is to study extremely low frequency (ELF) electromagnetic waves with elements drifting in frequency. It is shown that only some of them can be considered as usual chorus. These chorus elements are emitted in the equatorial plane, and their propagation analysis indicates that they are going downward at low altitudes in the ionosphere to be detected by the satellite. The study of one remarkable event recorded along the same orbit in both the Northern and the Southern Hemispheres on 8 May 2008 indicates that this propagation mechanism is reinforced at the location of the ionospheric trough, which corresponds to the plasmapause at higher altitudes. It has been observed that usual chorus elements at low frequencies are always in a frequency band which overlaps with a hiss band limited by a frequency cutoff close to the proton gyrofrequency. Other drifting elements can be attributed to emissions triggered by PLHR (power line harmonic radiation). It means that without a high-resolution spectral analysis, chorus-like elements triggered by PLHR can be wrongly considered as natural chorus. These drifting elements can also appear as filamentary structures emerging at the upper frequencies of a hiss band or quasiperiodic emissions. There are events where the elements even have certain similarities to quasiperiodic emissions. The difference between these elements and the chorus emissions will be emphasized

Statistical analysis of VLF radio emissions triggered by power line harmonic radiation and observed by the low-altitude satellite DEMETER

International audienceDEMETER was a low-altitude satellite in operation between 2004 and 2010 in a circular polar orbit. One of its main scientific objectives was to study ionospheric perturbations related to man-made activity. This paper investigates electromagnetic emissions triggered by Power Line Harmonic Radiation (PLHR), the man-made waves emitted at harmonics of 50 or 60 Hz. They look like rising tones or hooks with a starting frequency associated to a parent line with the frequency equal to a multiple of 50 or 60 Hz. They occur preferentially during daytime in a frequency band between 1 and 4 kHz. It is shown that these emissions are rather frequent at high latitudes (3 < L <6) above industrialized areas during periods of moderate magnetic activity. Their average intensity is of the order of 10 μV 2 m À2 Hz À1. PLHR propagates in the magnetosphere and triggers emissions due to wave-particle interactions in the equatorial region