Morphology and diagnostic potential of the ionospheric Alfvén resonator

The layering of the ionosphere leads to the formation of resonators and waveguides of various kinds. One of the most well-known is the ionospheric Alfvén resonator (IAR) whose radiation can be observed both on Earth’s surface and in space in the form of a fan-shaped set of discrete spectral bands (DSB), the frequency of which changes smoothly during the day. The bands are formed by Alfvén waves trapped between the lower part of the ionosphere and the altitude profile bending of Alfvén velocity in the transition region between the ionosphere and the magnetosphere. Thus, IAR is one of the important mechanisms of the ionosphere-magnetosphere interaction. The emission frequency lies in the range from tenths of hertz to about 8 Hz — the frequency of the first harmonic of the Schumann resonance. The review describes in detail the morphology of the phenomenon. It is emphasized that the IAR emission is a permanent phenomenon; the probability of observing it is primarily determined by the sensitivity of the equipment and the absence of interference of natural and artificial origin. The daily duration of the DSB observation almost completely depends on the illumination conditions of the lower ionosphere: the bands are clearly visible only when the D layer is shaded. Numerous theoretical IAR models have been systematized. All of them are based on the analysis of the excitation and propagation of Alfvén waves in inhomogeneous ionospheric plasma and differ mainly in sources of oscillation generation and methods of accounting for various factors such as interaction of wave modes, dipole geometry of the magnetic field, frequency dispersion of waves. Predicted by all models of the cavity and repeatedly confirmed experimentally, the close relationship between DSB frequency variations and critical frequency foF2 variations serves as the basis for searching ways of determining in real time the electron density of the ionosphere from IAR emission frequency measurements. It is also possible to estimate the profile of the ion composition over the ionosphere from the data on the IAR emission frequency structure. The review also focuses on other results from a wide range of IAR studies, specifically on the results that revealed the influence of the interplanetary magnetic field orien tation on oscillations of the resonator, and on the facts of the influence of seismic disturbances on IAR.

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Propagation of kinetic Alfvén waves in the ionospheric Alfvén resonator in the presence of density cavities

Geophysical Research Letters ◽

10.1029/2008gl035728 ◽

2008 ◽

Vol 35 (20) ◽

Cited By ~ 38

Author(s):

R. L. Lysak ◽

Y. Song

Keyword(s):

Alfven Waves ◽

Alfvén Waves ◽

Ionospheric Alfvén Resonator ◽

Alfven Resonator ◽

Ionospheric Alfven Resonator ◽

Kinetic Alfvén Waves

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Excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator by modulated powerful radio waves

Journal of Atmospheric and Solar-Terrestrial Physics ◽

10.1016/s1364-6826(99)00121-2 ◽

2000 ◽

Vol 62 (4) ◽

pp. 267-276 ◽

Cited By ~ 23

Author(s):

V.Y Trakhtengerts ◽

P.P Belyaev ◽

S.V Polyakov ◽

A.G Demekhov ◽

T Bösinger

Keyword(s):

Alfven Waves ◽

Alfvén Waves ◽

Radio Waves ◽

Powerful Radio ◽

Ionospheric Alfvén Resonator ◽

Alfven Resonator ◽

Ionospheric Alfven Resonator

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

Annales Geophysicae ◽

10.5194/angeo-23-499-2005 ◽

2005 ◽

Vol 23 (2) ◽

pp. 499-507 ◽

Cited By ~ 4

Author(s):

V. V. Alpatov ◽

M. G. Deminov ◽

D. S. Faermark ◽

I. A. Grebnev ◽

M. J. Kosch

Keyword(s):

Pulse Duration ◽

Alfven Waves ◽

Fundamental Period ◽

Alfvén Waves ◽

Shear Mode ◽

Q Factor ◽

Trapped Waves ◽

Alfven Resonator ◽

Ionospheric Alfven Resonator ◽

The Waves

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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Field-aligned structure of poloidal Alfvén waves in a finite pressure plasma

Annales Geophysicae ◽

10.5194/angeo-27-3875-2009 ◽

2009 ◽

Vol 27 (10) ◽

pp. 3875-3882 ◽

Cited By ~ 20

Author(s):

P. N. Mager ◽

D. Yu. Klimushkin ◽

V. A. Pilipenko ◽

S. Schäfer

Keyword(s):

Magnetic Field ◽

Cold Plasma ◽

Alfven Waves ◽

Parallel Structure ◽

Alfvén Waves ◽

Pressure Plasma ◽

Numerical Studies ◽

Transparent Region ◽

Alfven Resonator ◽

Northern And Southern Hemispheres

Abstract. This paper is devoted to analytical and numerical studies of the parallel structure of poloidal Alfvén waves in a finite pressure plasma. The effects of finite-beta plasma are especially essential near the magnetospheric equator, where an opaque (non-transparent) region for Alfvén waves can be formed. This region is bounded by two turning points which restrain penetration of the wave energy far from the ionosphere, and an Alfvén resonator appears on a part of the field line adjacent to the ionosphere. Due to this effect the ULF pulsations in the Northern and Southern Hemispheres can be non-conjugated. Another result is a peculiar field-aligned structure of the wave magnetic field: its fundamental harmonic must have three nodes, rather than one node as is the case in cold plasma.

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