Whistler Mode Waves Below Lower Hybrid Resonance Frequency: Generation and Spectral Features

Equatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft. We develop a model of this wave phenomenon which assumes (as, in general, has been suggested long ago) that the observed spectrum is excited due to loss-cone instability of energetic ions in the equatorial region of the magnetosphere. The wave field is represented as a sum of constant frequency wave packets which cross a number of cyclotron resonances while propagating in a highly oblique mode along quite specific trajectories. The growth (damping) rate of these wave packets varies both in sign and magnitude along the ray path, making the wave net amplification, but not the growth rate, the main characteristic of the wave generation process. The growth rates and the wave amplitudes along the ray paths, determined by the equations of geometrical optics, have been calculated for a 3D set of wave packets with various frequencies, initial L-shells, and initial wave normal angles at the equator. It is shown that the dynamical spectrum resulting from the proposed model qualitatively matches observations

Experimental determination of the dispersion relation of magnetosonic waves

Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory

Study of the lower hybrid resonance frequency over the regions of gathering earthquakes using DEMETER data

International audienceVariations of plasma distribution and/or wave spectral features in the ionosphere were suggested by many authors as possible earthquake precursors, and the change of plasma density and temperature above seismic regions were reported in the literature. These quantities are known to influence the lower hybrid resonance (LHR) frequency profiles in the upper ionosphere and the magnetosphere, which, in turn, strongly affects the propagation of quasi-resonance VLF waves with frequencies f close to the maximum of the LHR frequency on the propagation path. This makes the VLF signals a tool of registration of ionospheric perturbations. Using the measurements from the DEMETER satellite for 3 yr we have calculated the maps of LHR frequency over the globe, and the maps of VLF spectral intensity at the frequencies of Alpha navigation transmitters. These maps demonstrate a significant dependence of the spectral intensity in the transmitter conjugate region on the relation between the signal frequency and the LHR frequency above the observation point. Then, using the DEMETER data and the earthquake database from the US geological survey server we have performed statistical analysis of the LHR frequency over seismic regions and found an appreciably different behaviour of the LHR frequency before earthquakes, as compared to its regular behaviour, for several seismic regions. Although this difference is statistically significant, in each particular case the ionospheric perturbations may be related to different processes in the Earth's atmosphere, ionosphere, and the magnetosphere, other than gathering earthquakes. Thus, the unexpected variations in the LHR frequency profile, revealed from the variations of VLF transmitter signals, should only be considered as one indicator in a list of possible earthquake precursors

On the origin of lower- and upper-frequency cutoffs on wedge-like spectrograms observed by DEMETER in the midlatitude ionosphere

International audienceWe report observations of unusual lower and upper cutoff frequencies on VLF spectrograms recorded by the DEMETER satellite (orbiting at ∼700 km) during thunderstorm activity. The upper cutoff frequencies in the spectrograms under discussion vary rapidly, approximately in proportion to L −3 , where L is the McIlwain parameter on the satellite orbit. On the contrary, the lower cutoff frequencies in the spectrograms are almost constant, so that the cutoffs cross at larger L. Between these cutoffs, which thus form a wedge, intense whistlers are observed, whereas only 0 + whistlers, and probably, ducted whistlers are present outside the cutoffs. Using a model of lower hybrid resonance (LHR) frequency and local measurements of plasma density and ion composition in the ionosphere, we show that during the events under consideration, the satellite is located at altitudes where the height-dependent variation of the LHR frequency presents a trough. We explain the observed spectrograms on the basis of the features of whistler-mode wave propagation in the inner magnetosphere. Then the upper cutoff frequency is determined by the limiting trajectories that confine the waves of a given frequency propagating obliquely from a source located in the opposite hemisphere. Since the waves close to the limiting ones propagate in the quasiresonance regime, the intensity and time delay at the upper cutoff should increase, which is the case in observations. As for the lower frequency cutoff, it is determined by the LHR maximum, since quasiresonant waves with lower frequencies originating in the opposite hemisphere do not reach the satellite due to the LHR reflection above it