«Physics of the Auroral Phenomena», Proc. XXV Annual Seminar, Apatity, pp.81-84, 2002 Kola Science Centre, Russian Academy of Science, 2002 81 Polar Geophysical Institute VLF CHORUS CHARACTERISTICS AND PREDICTIONS FROM BACKWARD WAVE OSCILLATOR MODEL: A COMPARISON E. E. Titova, B. V. Kozelov (Polar Geophysical Institute, Apatity, Russia) F. Jiriček, J. Smilauer (Institute of Atmospheric Physics, Prague, Czech Republic) A. G. Demekhov, V. Yu. Trakhtengerts (Institute of Applied Physics, Nizhny Novgorod, Russia) Abstract. We present a study of chorus emissions in the magnetosphere detected onboard Magion 5, when the satellite was not far from the magnetic equator. We determined the frequency sweep rate of more than 8500 electromagnetic VLF chorus elements. The comparison of the observed chorus characteristics with the backward wave oscillator regime (BWO) of the chorus generation mechanism shows both qualitative and quantitative agreement with BWO model. Introduction Generation of chorus emissions is one of the most puzzling problems of VLF waves in the Earth magnetosphere. These emissions are the most intense of all natural VLF waves in the frequency range from a few hundred Hz to several kHz. They are observed as a succession of repeating discrete elements with rising frequency. It is generally accepted that the chorus is generated in a near-equatorial region by the cyclotron instability of radiation belt electrons (Helliwell, 1965). However, mechanisms responsible for the origin of chorus succession and formation of spectrum of separate chorus elements both are still unclear. Recently, Trakhtengerts (1999) suggested a mechanism of chorus generation based on the backward wave oscillator (BWO) regime of magnetospheric cyclotron maser (Trakhtengerts, 1995). The BWO regime of chorus generation gives us a hope to explain such features of chorus as appearance of a succession of discrete elements and their spectrum, relation of chorus to ELF hiss, large growth rates of chorus, and different generation regimes such as quasi-periodic and stochastic ones. The suggested BWO model of chorus generation allows for a number of predictions that can be checked experimentally. In this paper we compare spectral and amplitude characteristics of chorus observed onboard the MAGION 5 satellite with those predicted by the backward wave oscillator model of chorus generation. The BWO regime of chorus generation in the magnetosphere In this section, we briefly review the BWO regime of whistler wave generation in the magnetosphere (Trakhtengerts 1995, 1999) to select the parameters that can be compared with experimental data. This regime is similar to the backward wave oscillator in laboratory electronic devices where the wave propagates opposite to the motion of an interacting electron (Ginzburg and Kuznetsov, 1981). Similar waveparticle interactions take place in the magnetospheric cyclotron maser. The generation of chorus is based on the cyclotron resonance of radiation belt electrons with whistler waves ω  ω H = k || v || , (1) where ω is the wave frequency, ω H is the electron gyrofrequency, k || and v || are the magnetic field-aligned components of the wave vector and electron velocity. Certain conditions have to be satisfied for a generator to operate in the BWO regime. The first condition requires that the phase velocity component along the magnetic field should be opposite to the electron motion. According to (1), this condition is satisfied if ω < ω H . The second condition is the existence of a well-organized electron beam with small velocity dispersion in the region of chorus generation. This condition poses a significant problem, since there is no obvious reason for such a beam to be formed. The solution of this problem can be related to the fact that cyclotron interaction of band-limited natural ELF/VLF noise-like emissions with energetic electrons results in formation of a specific step-like feature of the distribution function (Bespalov, 1986, Nunn and Sazhin, 1991). This step-like deformation of energetic electron distribution function ensures large growth rate γ HD of whistler waves and transition to the BWO regime. Trakhtengerts (1995) showed that the step-like deformation of the distribution function, caused by interactions of natural ELF/VLF noise-like emissions and energetic electrons, acts in the magnetosphere as a well-organized beam in laboratory devices. The magnetospheric BWO has no fixed boundaries, and its interaction length l is determined by the inhomogeneity of the geomagnetic field. According to Helliwell (1967) and Trakhtengerts (1995), the interaction length l of whistler waves and energetic electrons can be written for the dipole magnetic field as follows: l= (R 0 2 L 2 /k) 1/3 , (2) where R 0 is the Earth’s radius, L is the geomagnetic shell, and k is the whistler wave number. The BWO generation starts when the density of energetic electrons exceeds some threshold value. According to Trakhtengerts (1995), this threshold condition can be written as p = 2 γ HD l / [π (V || V g ) 1/2 ] = 1, (3) where l is the working length of the magnetospheric generator, V g is the group velocity of the whistler