Abstract. 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.

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Abstract. We present the results of a case study of quasiperiodic (QP) ELF/VLF hiss emissions detected on board the Freja and Magion 5 satellites. Detailed analysis of available QP events revealed certain specific features of their dynamic spectra, which have not been reported earlier. In particular, we found an event with an increase in the frequency drift rate during the generation of a single element of QP emission, and an event with alteration of QP elements having different frequency drift rates. Another event demonstrates the possible relationship between QP hiss emissions and discrete VLF emissions. Properties of QP events are compared with parameters of energetic electrons and cold plasma, and other available data.Possible scenarios for the formation of these emissions are discussed on the basis of self-consistent simulations of the cyclotron instability, employing the information obtained experimentally. It is shown that the generation regime of selfsustained pulsations can explain consistently our data set. We show that our numerical model is capable of explaining the mentioned specific features of the dynamic spectrum of QP emissions. Comparison of the modeling results with experimental data yields an estimate for the parameters not measured directly.

Abstract.We study the penetration of lightning induced whistler waves through the ionosphere by investigating the correspondence between the whistlers observed on the DEMETER and MAGION-5 satellites and the lightning discharges detected by the European lightning detection network EUCLID. We compute all the possible differences between the times when the whistlers were observed on the satellite and times when the lightning discharges were detected. We show that the occurrence histogram for these time differences exhibits a distinct peak for a particular characteristic time, corresponding to the sum of the propagation time and a possible small time shift between the absolute time assigned to the wave record and the clock of the lightning detection network. Knowing this characteristic time, we can search in the EUCLID database for locations, currents, and polarities of causative lightning discharges corresponding to the individual whistlers. We demonstrate that the area in the ionosphere through which the electromagnetic energy induced by a lightning discharge enters into the magnetosphere as whistler mode waves is up to several thousands of kilometres wide.

Abstract. We present a detailed study of chorus emissions in the magnetosphere detected on board Magion 5, when the satellite was not far from the magnetic equator. We determine the frequency sweep rate of more than 8500 electromagnetic VLF chorus elements. These results are compared with the backward wave oscillator (BWO) regime of chorus generation. Comparison of the frequency sweep rate with the BWO model shows: (i) There is a correlation between the frequency sweep rates and the chorus amplitudes. The frequency sweep rate increases with chorus amplitude, in accordance with expectations from the BWO model; (ii) The chorus growth rate, estimated from the frequency sweep rate, is in accord with that inferred from the BWO generation mechanism; (iii) The BWO regime of chorus generation ensures the observed decrease in the frequency sweep rate of the chorus elements with increasing L-shell.Key words. Magnetospheric physics (VLF emissions, energetic particles) – Space plasma physics (wave-particle interactions)

The effects of broadening of frequency spectra of quasi‐monochromatic very low frequency (VLF) signals were observed in the polar ionosphere when VLF transmitter signals were registered on the satellite "Intercosmos‐19". Broadening of the spectrum was mainly registered on the electric antenna to the extent of Δf ≤ ± 1 kHz. Broadening of the VLF signal and intensive extra low frequency (ELF) noise with f < 1 kHz were observed simultaneously. Such effects can be explained by a transformation of the initial whistler‐wave, into whistler‐mode waves with wave‐normals very near the resonant cone, by small‐scale ionospheric irregularities. These irregularities can be caused by ion‐acoustic or ion‐cyclotron waves. The effect of the broadening of the VLF signal spectrum can be used as a diagnostic of ionospheric turbulence and field‐aligned electric currents in the upper ionosphere.

[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.

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