Dynamics of Alfvén waves in the night-side ionospheric Alfvén resonator at mid-latitudes

• Main Authors: V. V. Alpatov, M. G. Deminov, D. S. Faermark, I. A. Grebnev, M. J. Kosch
• Format: Article
• Language: English
• Published: Copernicus Publications 2005-02-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/23/499/2005/angeo-23-499-2005.pdf
• ISSN: 0992-7689; 1432-0576

A numerical solution of the problem on dynamics of shear-mode Alfv&#233;n waves in the ionospheric Alfv&#233;n resonator (IAR) region at middle latitudes at nighttime is presented for a case when a source emits a single pulse of duration &tau; into the resonator region. It is obtained that a part of the pulse energy is trapped by the IAR. As a result, there occur Alfv&#233;n 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 &tau; and it is maximum at &tau;=(3/4)<i>T</i>, where <i>T</i> 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 Alfv&#233;n 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 <i>T</i>, irrespective of the pulse duration &tau;. The characteristic time of damping of the trapped waves &tau;_dec is proportional to <i>T</i>, therefore the resonator Q-factor for such waves is independent of <i>T</i>. For a periodic source the amplitude-frequency characteristic of the IAR has a local minimum at the frequency &pi;/&omega;=(3/4)<i>T</i>, and the waves of such frequency do not accumulate energy in the resonator region. At the fundamental frequency &omega;=2&pi;/<i>T</i> 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 Alfv&#233;n waves. Explicit dependences of the IAR characteristics (<i>T</i>, &tau;_dec, Q-factor and eigenfrequencies) on the altitudinal distribution of Alfv&#233;n velocity are presented which are analytical approximations of numerical results.