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Ann. Geophys., 22, 1053-1060, 2004
www.ann-geophys.net/22/1053/2004/
© European Geosciences Union 2004
The spatio-temporal structure of impulse-generated azimuthalsmall-scale Alfvén waves interacting with high-energy chargedparticles in the magnetosphere
D. Yu. Klimushkin and P. N. Mager
Institute of Solar-Terrestrial Physics (ISTP), Russian Academy of Science, Siberian Branch, Irkutsk, P.O.Box 4026, 664033, Russia
Abstract. It is assumed to date that the energy source of
azimuthal small-scale ULF waves in the magnetosphere (azimuthal
wave numbers m≧1) is provided by the energetic particles
interacting with the waves through the bounce-drift resonance.
In this paper we
have solved the problem of the bounce-drift instability influence
on the spatio-temporal structure of Alfvén waves excited by
a source of the type of sudden impulse in a dipole-like
magnetosphere. It is shown that the impulse-generated Alfvén
oscillation within a time τ~m∕ΩTN (where
ΩTN is the toroidal eigenfrequency) is a poloidal one,
and each field line oscillates with its own eigenfrequency that
coincides with the poloidal frequency of a given L-shell.
As time elapses, the wave becomes toroidally polarized because
of the phase difference of the disturbance, and the oscillation
frequency of field lines tends to the toroidal frequency.
The drift-bounce instability growth rate becomes smaller
during the wave temporal evolution, and the instability
undergoes stabilization when the wave frequency coincides
with the toroidal eigenfrequency. The total amplification of
the wave can be estimated as
 ,
where
 is the wave growth rate at the beginning
of the process, when it has its maximum value.
The wave amplitude can increase only within a time
~τ, when it is poloidally polarized. After this time, when the
wave becomes to be toroidally polarized, it goes damped because
of the finite ionospheric conductivity. This is in qualitative
agreement with the recent radar experimental data.
Key words. Magnetospheric physics (MHD waves and instabilities).
Space plasma physics (kinetic and MHD theory;
wave-particle interactions)
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