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Ann. Geophys., 23, 499-507, 2005
www.ann-geophys.net/23/499/2005/
© European Geosciences Union 2005
Dynamics of Alfvén waves in the night-side ionospheric Alfvén resonator at mid-latitudes
V. V. Alpatov1, M. G. Deminov2, D. S. Faermark1, I. A. Grebnev1, and M. J. Kosch3
1Institute of Applied Geophysics, Rostokinskaya st., 9, Moscow, 129128, Russia
2Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, RAS, Troitsk, Moscow region, 142190, Russia
3Department of Communication Systems, Lancaster University, LA1 4YR Lancaster, United Kingdom
Abstract. A numerical solution of the problem on dynamics of
shear-mode Alfvén waves in the ionospheric Alfvén resonator (IAR) region at
middle latitudes at nighttime is presented for a case when a source emits a
single pulse of duration τ into the resonator region. It is obtained
that a part of the pulse energy is trapped by the IAR. As a result, there
occur Alfvén waves trapped by the resonator which are being damped. It is
established that the amplitude of the trapped waves depends essentially on
the emitted pulse duration τ and it is maximum at τ=(3/4)T,
where T is the IAR fundamental period. The maximum amplitude of these waves
does not exceed 30% of the initial pulse even under optimum conditions.
Relatively low efficiency of trapping the shear-mode Alfvén waves is caused
by a difference between the optimum duration of the pulse and the fundamental
period of the resonator. The period of oscillations of the trapped waves is
approximately equal to T, irrespective of the pulse duration τ. The
characteristic time of damping of the trapped waves τdec is
proportional to T, therefore the resonator Q-factor for such waves is
independent of T. For a periodic source the amplitude-frequency
characteristic of the IAR has a local minimum at the frequency π/ω=(3/4)T, and the waves of such frequency do not accumulate
energy in the resonator region. At the fundamental frequency ω=2π/T
the amplitude of the waves coming from the periodic source can be
amplified in the resonator region by more than 50%. This alone is a basic
difference between efficiencies of pulse and periodic sources of Alfvén
waves. Explicit dependences of the IAR characteristics (T, τdec, Q-factor
and eigenfrequencies) on the altitudinal distribution of Alfvén velocity are
presented which are analytical approximations of numerical results.
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