24 research outputs found
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
Characteristic properties of Nu whistlers as inferred from observations and numerical modelling
The properties of Nu whistlers are discussed in the light of observations
by the MAGION 5 satellite, and of numerically simulated spectrograms of
lightning-induced VLF emissions. The method of
simulation is described in full. With the information
from this numerical modelling, we distinguish the characteristics of the
spectrograms that depend on the site of the lightning strokes from those
that are determined mainly by the position of the satellite.
Also, we identify the region in the magnetosphere where Nu whistlers
are observed most often, and the geomagnetic conditions favouring their
appearance. The relation between magnetospherically reflected (MR)
whistlers and Nu whistlers is demonstrated by the gradual transformation
of MR whistlers into Nu whistlers as the satellite moves from
the high-altitude equatorial region to lower altitudes and higher latitudes.
The magnetospheric reflection of nonducted whistler-mode waves,
which is of decisive importance in the formation
of Nu whistlers, is discussed in detail
Magion 5 observations of chorus-like emissions and their propagation features as inferred from ray-tracing simulation
After reviewing briefly the present state of
knowledge about chorus-like emissions, we present an overview of Magion 5
satellite observations of these emissions in the inner magnetosphere of the
Earth. From the extensive VLF data recorded on board the Magion 5 satellite, we
show examples of different types of discrete elements, representing rising and
falling tones, and discuss their spectral properties, such as the bandwidth and
the characteristic frequency as compared to the equatorial electron
gyrofrequency. We analyse the possibility of satellite observation of discrete
elements, assuming nonducted wave propagation from the source. As for the
characteristic dimension of the generation region, we apply the figures
obtained from the recently published correlation analysis of chorus emission
recorded by four satellites in the Cluster experiment. We conclude that
different frequencies in the chorus element should be emitted in a certain span
of wave normal angles, so that the whole element could be observed far from the
generation region.Key words. Magnetospheric physics (plasmasphere;
plasma waves and instabilities) – Space plasma physics (wave-particle interactions)
– Ionosphere (wave propagation