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It is assumed to date that the energy source of\udazimuthal small-scale ULF waves in the magnetosphere (azimuthal\udwave numbers m&#x2267;1) is provided by the energetic particles\udinteracting with the waves through the bounce-drift resonance.\udIn this paper we\udhave solved the problem of the bounce-drift instability influence\udon the spatio-temporal structure of Alfvén waves excited by\uda source of the type of sudden impulse in a dipole-like\udmagnetosphere. It is shown that the impulse-generated Alfvén\udoscillation within a time &#x03C4;~m&#x2215;&#x03A9;TN (where\ud\ud&#x03A9;TN is the toroidal eigenfrequency) is a poloidal one,\udand each field line oscillates with its own eigenfrequency that\udcoincides with the poloidal frequency of a given L-shell.\udAs time elapses, the wave becomes toroidally polarized because\udof the phase difference of the disturbance, and the oscillation\udfrequency of field lines tends to the toroidal frequency.\udThe drift-bounce instability growth rate becomes smaller\udduring the wave temporal evolution, and the instability\udundergoes stabilization when the wave frequency coincides\udwith the toroidal eigenfrequency. The total amplification of\udthe wave can be estimated as \ud<img src="ag-22-1053-img5.gif" alt="$e^{tilde{gamma}tau}$" align="bottom" border="0" height="16" width="26">,\udwhere \ud<img src="ag-22-1053-img6.gif" alt="$tilde{gamma}$" align="middle" border="0" height="30" width="12"> is the wave growth rate at the beginning\udof the process, when it has its maximum value.\udThe wave amplitude can increase only within a time\ud~&#x03C4;, when it is poloidally polarized. After this time, when the\udwave becomes to be toroidally polarized, it goes damped because\udof the finite ionospheric conductivity. This is in qualitative\udagreement with the recent radar experimental data. Key words. Magnetospheric physics (MHD waves and instabilities).\udSpace plasma physics (kinetic and MHD theory;\udwave-particle interactions

annales geophysicae

The spatio-temporal structure of impulse-generated azimuthalsmall-scale Alfvén waves interacting with high-energy chargedparticles in the magnetosphere