A new generation regime of the magnetosphere cyclotron maser is considered, based on phase coherence effects in wave-particle systems with step-like deformations of electron velocity distribution functions. Such deformations appear during cyclotron interactions of noiselike whistler wave emissions and energetic electrons at the boundaries between resonant and nonresonant particles. The new regime is similar to the backward wave oscillator in laboratory electronic devices. This regime applies to the generation of chorus emissions and may explain the connection between chorus and hiss, their fast growth rates, and the temporal succession of chorus elements.peculiarities of phase and group trajectories. This problem to be fulfilled for the BWO generation regime to operate. The seemed to be solved in frame of MCM concept. However, the main one is a well-organized beam of electrons with small farst attempts were unsuccessful. It has turned out that velocity dispersion, rotating in the magnetic field. The second observable, one-pass amplification of chorus signals was stronger condition is a sufficiently long system or a large beam current. than theoretical estimations suggested. It has been found to be Under magnetospheric conditions, well-organized electron impossible to connect a temporal consequence of chorus signals beams with small velocity dispersion are very rare. with the known periods of MCM such as wave packet group Occasionally such beams can appear in the polar regions during delay between conjugate ionospheres or electron bounce substorm events. Probably such beams may also be formed under the action of VLF-transmitter signals or whistlers from lightning discharges. However, it is difficult to assume that such Copyright 1995 by the American Geophysical Union. beams exist regularly in chorus-generation regions. Paper number 95JA00843. Below we show that the BWO generation regime is possible 0148-0227/95/95JA-00843505.00 not only for electron beams with a • function distribution in 17,205 17,206 TRAKHTENGERTS: MAGNETOSPHERE CYCLOTRON MASER electron velocity space but also in the case of distribution functions with step-like deformations in velocity space. Such distribution functions are formed in the magnetosphere naturally under the action of noise-like ELF emission. We then apply our

The subject of wave-particle interactions occurring in space plasmas has developed strongly, both observationally and theoretically, since the discovery of the Van Allen radiation belts of energetic charged particles trapped in the Earth's magnetosphere over forty years ago. These wave-particle interactions are recognized today as being a most important research topic in space plasma physics. This book provides a full and systematic description of the physical theory of whistler and Alfvén cyclotron masers acting in planetary magnetospheres, and in the Sun's outer atmosphere. The book introduces research topics by examining significant problems in the subject. It gives sufficient detail on the topic that readers can go on to apply the methods presented to new problems, helping them with their own research. This book is a valuable reference for researchers and graduate students working in space science, solar-terrestrial physics, plasma physics, and planetary sciences.

Abstract. A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.Key words. Magnetospheric physics (Energetic particles · trapped). Space plasma physics (wave-particle interactions; waves and instabilities)

We consider a mechanism of pulsating aurora formation relevant for the substorm recovery phase. It is based on the spikelike regime of whistler cyclotron instability in auroral ducts with enhmacecl plasma density, elongated with geomagnetic field lines. We present the rigorously derived self-consistent quasi-linear theory of flow cyclotron maser operation, which describes spikelike regimes of whistler wave generation and energetic particle precipitation in a duct. We show that pu]m•tions are formed due to broadening of the resonance region in velocity space in the course of irmtabillty development. We have obtained theoretical estimations for temporal characteristics of pu]m•ing auroral patches. Pulsating regime is realized when the energetic electron number density is below some critical value depending on background plasma density and particle source paratneters. Pulsation "on" time is determined by the nonlinear instability dynamics whereas "off" time is nearly equal to the time of reaching the wave excitation threshold. We compare our analytical and numerical results with known experimental data and show that the model explains reasonably most observations concerning pulsation time pattern, electron precipitation fluxes, VLF wave spectIaxm dynamics, latitudinal and longitudinal dependencies of pulsation characteristics, etc. We have made also preliminary estimations of nonlinear particle trapping effects during the maximum of the on phase that can give rise to a fine temporal structure of pulsations.

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 and spatial characteristics close to the Cluster data. Amplitudes and frequency spectra, as well as dynamical features of the Poynting flux of chorus are estimated and compared with the Cluster measurements.