How energetic particles construct and destroy poloidal high- [formula omitted] Alfvén waves in the magnetosphere

Abstract

The paper overviews the role of energetic particles in generation of poloidal ULF waves with high azimuthal wave numbers ( m ⪢ 1 ) . The part played by these particles is twofold. First, they influence the wave polarization. The presence of the energetic plasma component leads to appearance of the additional (ballooning) terms in the wave equations, that influences the value of the poloidal frequency, while the toroidal frequency is determined mainly by the density of the cold plasma component. Transverse Alfvén waves can be poloidally polarized provided that the difference between these two frequencies is large enough. Thus, the increasing of the energetic plasma component density is beneficial for the poloidal polarization of the wave. The second effect of the energetic particles on the Alfvén waves is the drift-bounce resonance, which can excite the oscillations. This instability is in competition with the damping caused by the interaction of the waves with the ionosphere. A spatio–temporal structure of the oscillations generated by the instability is considered. Monochromatic waves are confined between the so-called toroidal and poloidal surfaces. The wave is generated near the poloidal surface and is propagating in the direction of the poloidal surface, changing the polarization from poloidal to toroidal. While the local growth rate is a maximum on the poloidal surface, the result of the instability, the amplitude maximum takes place near the toroidal surface. Thus, the toroidally rather than poloidally polarized oscillations are the most enhanced ones. This is also true for the impulse-generated oscillations. The ionospheric finite resistance can prevent this transformation, if the corresponding damping rate is larger than the instability growth rate.